Begin integrating support for themes (#1367)

packages/desktop-client/src/components/App.js

packages/desktop-client/src/components/ThemeSelector.tsx

packages/desktop-client/src/components/Titlebar.js

...es/desktop-client/src/components/settings/Experimental.js

...s/desktop-client/src/components/settings/Experimental.tsx

packages/desktop-client/src/components/settings/Themes.js

packages/desktop-client/src/components/settings/index.js

packages/desktop-client/src/hooks/useFeatureFlag.ts

packages/desktop-client/src/icons/v2/MoonStars.js

packages/desktop-client/src/icons/v2/Sun.js

packages/desktop-client/src/icons/v2/moon-stars.svg

packages/desktop-client/src/icons/v2/sun.svg

packages/desktop-client/src/style/colors.ts

packages/desktop-client/src/style/index.ts

packages/desktop-client/src/style/palette.ts

packages/desktop-client/src/style/styles.ts

packages/desktop-client/src/style/theme.tsx

packages/desktop-client/src/style/themes/dark.ts

packages/desktop-client/src/style/themes/development.ts

packages/desktop-client/src/style/themes/light.ts