feat(web): add theme definition system with 5 built-in themes (#493)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/themes/index.ts

@@ -0,0 +1,18 @@
+export type { ThemeColors, ThemeDefinition, ThemeShadows, ThemeColorKey } from "./types";
+export { themeToVariables } from "./types";
+
+export { darkTheme } from "./dark";
+export { lightTheme } from "./light";
+export { nordTheme } from "./nord";
+export { draculaTheme } from "./dracula";
+export { solarizedDarkTheme } from "./solarized-dark";
+
+export {
+  getAllThemes,
+  getTheme,
+  getThemeOrDefault,
+  getDarkThemes,
+  getLightThemes,
+  isValidThemeId,
+  DEFAULT_THEME_ID,
+} from "./registry";