diff --git a/apps/web/src/providers/ThemeProvider.test.tsx b/apps/web/src/providers/ThemeProvider.test.tsx
index 5d592b5..5ae6295 100644
--- a/apps/web/src/providers/ThemeProvider.test.tsx
+++ b/apps/web/src/providers/ThemeProvider.test.tsx
@@ -3,10 +3,12 @@ import { render, screen, act } from "@testing-library/react";
 import { ThemeProvider, useTheme } from "./ThemeProvider";
 
 function ThemeConsumer(): React.JSX.Element {
-  const { theme, resolvedTheme, setTheme, toggleTheme } = useTheme();
+  const { theme, themeId, themeDefinition, resolvedTheme, setTheme, toggleTheme } = useTheme();
   return (
     <div>
       <span data-testid="theme">{theme}</span>
+      <span data-testid="themeId">{themeId}</span>
+      <span data-testid="themeName">{themeDefinition.name}</span>
       <span data-testid="resolved">{resolvedTheme}</span>
       <button
         onClick={() => {
@@ -22,6 +24,27 @@ function ThemeConsumer(): React.JSX.Element {
       >
         Set Dark
       </button>
+      <button
+        onClick={() => {
+          setTheme("nord");
+        }}
+      >
+        Set Nord
+      </button>
+      <button
+        onClick={() => {
+          setTheme("dracula");
+        }}
+      >
+        Set Dracula
+      </button>
+      <button
+        onClick={() => {
+          setTheme("system");
+        }}
+      >
+        Set System
+      </button>
       <button
         onClick={() => {
           toggleTheme();
@@ -38,7 +61,9 @@ describe("ThemeProvider", (): void => {
 
   beforeEach((): void => {
     localStorage.clear();
-    document.documentElement.classList.remove("light", "dark");
+    document.documentElement.removeAttribute("data-theme");
+    // Clear any inline style properties set by theme application
+    document.documentElement.removeAttribute("style");
 
     mockMatchMedia = vi.fn().mockReturnValue({
       matches: false,
@@ -65,6 +90,7 @@ describe("ThemeProvider", (): void => {
     );
 
     expect(screen.getByTestId("theme")).toHaveTextContent("light");
+    expect(screen.getByTestId("themeId")).toHaveTextContent("light");
   });
 
   it("should NOT read from old 'jarvis-theme' storage key", (): void => {
@@ -76,7 +102,6 @@ describe("ThemeProvider", (): void => {
       </ThemeProvider>
     );
 
-    // Should default to system, not read from jarvis-theme
     expect(screen.getByTestId("theme")).toHaveTextContent("system");
   });
 
@@ -106,7 +131,6 @@ describe("ThemeProvider", (): void => {
   });
 
   it("should throw when useTheme is used outside provider", (): void => {
-    // Suppress console.error for expected error
     const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => {
       // Intentionally empty
     });
@@ -117,4 +141,201 @@ describe("ThemeProvider", (): void => {
 
     consoleSpy.mockRestore();
   });
+
+  it("should resolve 'system' to dark when OS prefers dark", (): void => {
+    mockMatchMedia.mockReturnValue({
+      matches: true,
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    });
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("dark");
+    expect(screen.getByTestId("resolved")).toHaveTextContent("dark");
+  });
+
+  it("should resolve 'system' to light when OS prefers light", (): void => {
+    mockMatchMedia.mockReturnValue({
+      matches: false,
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    });
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("light");
+    expect(screen.getByTestId("resolved")).toHaveTextContent("light");
+  });
+
+  it("should support Nord theme", (): void => {
+    localStorage.setItem("mosaic-theme", "nord");
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("theme")).toHaveTextContent("nord");
+    expect(screen.getByTestId("themeId")).toHaveTextContent("nord");
+    expect(screen.getByTestId("themeName")).toHaveTextContent("Nord");
+    expect(screen.getByTestId("resolved")).toHaveTextContent("dark");
+  });
+
+  it("should support Dracula theme", (): void => {
+    localStorage.setItem("mosaic-theme", "dracula");
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("dracula");
+    expect(screen.getByTestId("resolved")).toHaveTextContent("dark");
+  });
+
+  it("should support Solarized Dark theme", (): void => {
+    localStorage.setItem("mosaic-theme", "solarized-dark");
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("solarized-dark");
+    expect(screen.getByTestId("resolved")).toHaveTextContent("dark");
+  });
+
+  it("should fall back to system for unknown theme IDs", (): void => {
+    localStorage.setItem("mosaic-theme", "nonexistent-theme");
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    // Falls back to "system" because "nonexistent-theme" is not a valid theme ID
+    expect(screen.getByTestId("theme")).toHaveTextContent("system");
+  });
+
+  it("should switch between themes via setTheme", (): void => {
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    act(() => {
+      screen.getByText("Set Nord").click();
+    });
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("nord");
+    expect(screen.getByTestId("themeName")).toHaveTextContent("Nord");
+    expect(localStorage.getItem("mosaic-theme")).toBe("nord");
+
+    act(() => {
+      screen.getByText("Set Dracula").click();
+    });
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("dracula");
+    expect(localStorage.getItem("mosaic-theme")).toBe("dracula");
+  });
+
+  it("should toggle between dark and light", (): void => {
+    localStorage.setItem("mosaic-theme", "dark");
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("resolved")).toHaveTextContent("dark");
+
+    act(() => {
+      screen.getByText("Toggle").click();
+    });
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("light");
+    expect(screen.getByTestId("resolved")).toHaveTextContent("light");
+  });
+
+  it("should toggle from a dark theme (nord) to light", (): void => {
+    localStorage.setItem("mosaic-theme", "nord");
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("resolved")).toHaveTextContent("dark");
+
+    act(() => {
+      screen.getByText("Toggle").click();
+    });
+
+    expect(screen.getByTestId("themeId")).toHaveTextContent("light");
+    expect(screen.getByTestId("resolved")).toHaveTextContent("light");
+  });
+
+  it("should apply CSS variables on theme change", (): void => {
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    act(() => {
+      screen.getByText("Set Nord").click();
+    });
+
+    // Nord's bg-900 is #2e3440
+    const bgValue = document.documentElement.style.getPropertyValue("--ms-bg-900");
+    expect(bgValue).toBe("#2e3440");
+  });
+
+  it("should set data-theme attribute based on isDark", (): void => {
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    act(() => {
+      screen.getByText("Set Nord").click();
+    });
+
+    expect(document.documentElement.getAttribute("data-theme")).toBe("dark");
+
+    act(() => {
+      screen.getByText("Set Light").click();
+    });
+
+    expect(document.documentElement.getAttribute("data-theme")).toBe("light");
+  });
+
+  it("should expose themeDefinition with full theme data", (): void => {
+    localStorage.setItem("mosaic-theme", "dark");
+
+    render(
+      <ThemeProvider>
+        <ThemeConsumer />
+      </ThemeProvider>
+    );
+
+    expect(screen.getByTestId("themeName")).toHaveTextContent("Dark");
+  });
 });
diff --git a/apps/web/src/providers/ThemeProvider.tsx b/apps/web/src/providers/ThemeProvider.tsx
index a1a68b7..693d7c3 100644
--- a/apps/web/src/providers/ThemeProvider.tsx
+++ b/apps/web/src/providers/ThemeProvider.tsx
@@ -1,13 +1,35 @@
 "use client";
 
-import { createContext, useCallback, useContext, useEffect, useState, type ReactNode } from "react";
+import {
+  createContext,
+  useCallback,
+  useContext,
+  useEffect,
+  useMemo,
+  useState,
+  type ReactNode,
+} from "react";
 
-type Theme = "light" | "dark" | "system";
+import {
+  type ThemeDefinition,
+  darkTheme,
+  getThemeOrDefault,
+  isValidThemeId,
+  themeToVariables,
+} from "@/themes";
 
 interface ThemeContextValue {
-  theme: Theme;
+  /** User preference: a theme ID (e.g. "dark", "nord") or "system" */
+  theme: string;
+  /** The active theme's ID after resolving "system" */
+  themeId: string;
+  /** The full active ThemeDefinition object */
+  themeDefinition: ThemeDefinition;
+  /** "light" or "dark" classification of the active theme */
   resolvedTheme: "light" | "dark";
-  setTheme: (theme: Theme) => void;
+  /** Set theme by ID or "system" */
+  setTheme: (theme: string) => void;
+  /** Quick toggle between "dark" and "light" themes */
   toggleTheme: () => void;
 }
 
@@ -15,105 +37,112 @@ const ThemeContext = createContext<ThemeContextValue | null>(null);
 
 const STORAGE_KEY = "mosaic-theme";
 
-function getSystemTheme(): "light" | "dark" {
+function getSystemThemeId(): "light" | "dark" {
   if (typeof window === "undefined") return "dark";
   return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
 }
 
-function getStoredTheme(): Theme {
+function getStoredPreference(): string {
   if (typeof window === "undefined") return "system";
   const stored = localStorage.getItem(STORAGE_KEY);
-  if (stored === "light" || stored === "dark" || stored === "system") {
+  if (stored && (stored === "system" || isValidThemeId(stored))) {
     return stored;
   }
   return "system";
 }
 
-/**
- * Apply the resolved theme to the <html> element via data-theme attribute.
- * The default (no attribute or data-theme="dark") renders dark — dark is default.
- * Light theme requires data-theme="light".
- */
-function applyThemeAttribute(resolved: "light" | "dark"): void {
+function resolveThemeId(preference: string): string {
+  if (preference === "system") return getSystemThemeId();
+  return preference;
+}
+
+function applyThemeVariables(themeDef: ThemeDefinition): void {
   const root = document.documentElement;
-  if (resolved === "light") {
-    root.setAttribute("data-theme", "light");
-  } else {
-    // Remove the attribute so the default (dark) CSS variables apply.
-    root.removeAttribute("data-theme");
+  const vars = themeToVariables(themeDef);
+
+  for (const [prop, value] of Object.entries(vars)) {
+    root.style.setProperty(prop, value);
   }
+
+  // Set data-theme attribute for CSS selectors that depend on light/dark
+  root.setAttribute("data-theme", themeDef.isDark ? "dark" : "light");
 }
 
 interface ThemeProviderProps {
   children: ReactNode;
-  defaultTheme?: Theme;
+  defaultTheme?: string;
 }
 
 export function ThemeProvider({
   children,
   defaultTheme = "system",
 }: ThemeProviderProps): React.JSX.Element {
-  const [theme, setThemeState] = useState<Theme>(defaultTheme);
-  const [resolvedTheme, setResolvedTheme] = useState<"light" | "dark">("dark");
+  const [preference, setPreference] = useState<string>(defaultTheme);
   const [mounted, setMounted] = useState(false);
 
-  // Initialize theme from storage on mount
+  const themeId = useMemo(() => resolveThemeId(preference), [preference]);
+  const themeDefinition = useMemo(() => getThemeOrDefault(themeId), [themeId]);
+  const resolvedTheme = themeDefinition.isDark ? "dark" : "light";
+
+  // Initialize from storage on mount
   useEffect(() => {
     setMounted(true);
-    const storedTheme = getStoredTheme();
-    const resolved = storedTheme === "system" ? getSystemTheme() : storedTheme;
-    setThemeState(storedTheme);
-    setResolvedTheme(resolved);
-    applyThemeAttribute(resolved);
+    const stored = getStoredPreference();
+    setPreference(stored);
+
+    const id = resolveThemeId(stored);
+    const def = getThemeOrDefault(id);
+    applyThemeVariables(def);
   }, []);
 
-  // Apply theme via data-theme attribute on html element
+  // Apply theme whenever preference changes (after mount)
   useEffect(() => {
     if (!mounted) return;
+    applyThemeVariables(themeDefinition);
+  }, [themeDefinition, mounted]);
 
-    const resolved = theme === "system" ? getSystemTheme() : theme;
-    applyThemeAttribute(resolved);
-    setResolvedTheme(resolved);
-  }, [theme, mounted]);
-
-  // Listen for system theme changes
+  // Listen for system theme changes when preference is "system"
   useEffect(() => {
-    if (!mounted || theme !== "system") return;
+    if (!mounted || preference !== "system") return;
 
     const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
     const handleChange = (e: MediaQueryListEvent): void => {
-      const resolved = e.matches ? "dark" : "light";
-      setResolvedTheme(resolved);
-      applyThemeAttribute(resolved);
+      const id = e.matches ? "dark" : "light";
+      const def = getThemeOrDefault(id);
+      applyThemeVariables(def);
+      // Force re-render by updating preference to trigger useMemo recalc
+      setPreference("system");
     };
 
     mediaQuery.addEventListener("change", handleChange);
     return (): void => {
       mediaQuery.removeEventListener("change", handleChange);
     };
-  }, [theme, mounted]);
+  }, [preference, mounted]);
 
-  const setTheme = useCallback((newTheme: Theme) => {
-    setThemeState(newTheme);
-    localStorage.setItem(STORAGE_KEY, newTheme);
+  const setTheme = useCallback((newPreference: string) => {
+    setPreference(newPreference);
+    localStorage.setItem(STORAGE_KEY, newPreference);
   }, []);
 
   const toggleTheme = useCallback(() => {
     setTheme(resolvedTheme === "dark" ? "light" : "dark");
   }, [resolvedTheme, setTheme]);
 
-  // Prevent flash by not rendering until mounted
+  // SSR placeholder — render children but with dark defaults
   if (!mounted) {
     return (
       <ThemeContext.Provider
         value={{
           theme: defaultTheme,
+          themeId: "dark",
+          themeDefinition: darkTheme,
           resolvedTheme: "dark",
           setTheme: (): void => {
-            // No-op during SSR
+            /* no-op during SSR */
           },
           toggleTheme: (): void => {
-            // No-op during SSR
+            /* no-op during SSR */
           },
         }}
       >
@@ -123,7 +152,16 @@ export function ThemeProvider({
   }
 
   return (
-    <ThemeContext.Provider value={{ theme, resolvedTheme, setTheme, toggleTheme }}>
+    <ThemeContext.Provider
+      value={{
+        theme: preference,
+        themeId,
+        themeDefinition,
+        resolvedTheme,
+        setTheme,
+        toggleTheme,
+      }}
+    >
       {children}
     </ThemeContext.Provider>
   );
diff --git a/docs/TASKS.md b/docs/TASKS.md
index 3b61918..00ef2eb 100644
--- a/docs/TASKS.md
+++ b/docs/TASKS.md
@@ -5,8 +5,8 @@
 | id          | status      | description                                                                                                                                                | issue | repo | branch                       | depends_on                                             | blocks                                      | agent        | started_at | completed_at | estimate | used | notes                                      |
 | ----------- | ----------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------- | ----- | ---- | ---------------------------- | ------------------------------------------------------ | ------------------------------------------- | ------------ | ---------- | ------------ | -------- | ---- | ------------------------------------------ |
 | TW-PLAN-001 | done        | Plan MS18 task breakdown, create milestone + issues, populate TASKS.md                                                                                     | —     | —    | —                            |                                                        | TW-THM-001,TW-WDG-001,TW-EDT-001,TW-KBN-001 | orchestrator | 2026-02-23 | 2026-02-23   | 15K      | ~12K | Planning complete, all artifacts committed |
-| TW-THM-001  | in-progress | Theme architecture — Create theme definition interface, theme registry, and 5 built-in themes (Dark, Light, Nord, Dracula, Solarized) as TS files          | #487  | web  | feat/ms18-theme-architecture | TW-PLAN-001                                            | TW-THM-002,TW-THM-003                       | worker       | 2026-02-23 | —            | 30K      | —    |                                            |
-| TW-THM-002  | not-started | ThemeProvider upgrade — Load themes dynamically from registry, apply CSS variables, support instant theme switching without page reload                    | #487  | web  | TBD                          | TW-THM-001                                             | TW-THM-003,TW-VER-002                       | worker       | —          | —            | 25K      | —    |                                            |
+| TW-THM-001  | done        | Theme architecture — Create theme definition interface, theme registry, and 5 built-in themes (Dark, Light, Nord, Dracula, Solarized) as TS files          | #487  | web  | feat/ms18-theme-architecture | TW-PLAN-001                                            | TW-THM-002,TW-THM-003                       | worker       | 2026-02-23 | 2026-02-23   | 30K      | ~15K | PR #493 merged                             |
+| TW-THM-002  | in-progress | ThemeProvider upgrade — Load themes dynamically from registry, apply CSS variables, support instant theme switching without page reload                    | #487  | web  | TBD                          | TW-THM-001                                             | TW-THM-003,TW-VER-002                       | worker       | 2026-02-23 | —            | 25K      | —    |                                            |
 | TW-THM-003  | not-started | Theme selection UI — Settings page section with theme browser, live preview swatches, persist selection to UserPreference.theme via API                    | #487  | web  | TBD                          | TW-THM-001,TW-THM-002                                  | TW-VER-002                                  | worker       | —          | —            | 25K      | —    |                                            |
 | TW-WDG-001  | not-started | Widget definition seeding — Seed 7 existing widgets into widget_definitions table with correct sizing constraints and configSchema                         | #488  | api  | TBD                          | TW-PLAN-001                                            | TW-WDG-002                                  | worker       | —          | —            | 15K      | —    |                                            |
 | TW-WDG-002  | not-started | Dashboard → WidgetGrid migration — Replace hardcoded dashboard layout with WidgetGrid, load/save layout via UserLayout API, default layout on first visit  | #488  | web  | TBD                          | TW-WDG-001                                             | TW-WDG-003,TW-WDG-004,TW-WDG-005            | worker       | —          | —            | 40K      | —    |                                            |
@@ -23,12 +23,12 @@
 
 ## Summary
 
-| Metric        | Value             |
-| ------------- | ----------------- |
-| Total tasks   | 16                |
-| Completed     | 1 (PLAN-001)      |
-| In Progress   | 1 (THM-001)       |
-| Remaining     | 14                |
-| PRs merged    | —                 |
-| Issues closed | —                 |
-| Milestone     | MS18-ThemeWidgets |
+| Metric        | Value                 |
+| ------------- | --------------------- |
+| Total tasks   | 16                    |
+| Completed     | 2 (PLAN-001, THM-001) |
+| In Progress   | 1 (THM-002)           |
+| Remaining     | 13                    |
+| PRs merged    | —                     |
+| Issues closed | —                     |
+| Milestone     | MS18-ThemeWidgets     |