feat(FN-1214): add ember color theme

- Add ember theme to COLOR_THEMES array in types.ts
- Add ember theme CSS variables in styles.css
- Add ember theme swatch preview in ThemeSelector component
- Add ember to validThemes validation in index.html
- Add ThemeSelector tests for ember theme

packages/core/src/types.ts

@@ -54,6 +54,7 @@ export const COLOR_THEMES = [
   "espresso",
   "mars",
   "poimandres",
+  "ember",
 ] as const;
 export type ColorTheme = (typeof COLOR_THEMES)[number];