UI: Theme wheel

[akyoto]

Add a "theme wheel" to the bottom right corner. It is composed of colored circles arranged in a circular layout. It's hidden by default. When you click "Theme" in the sidebar, the wheel shows up, rotates to the next color and changes the theme to that color.

This is also a good chance to add more themes to the page, including a "Sakura" or "Crimson Red" theme.