Fixes selects having unreadable text on some browsers/OSes.

Currently some Browser/OS configurations allow <option> to be somewhat styled, and it leads to them being unreadable on darker themes due to color being inherited from body to a light color but background still being white. See image below:

Currently known browsers/os that has this issue:

• Firefox/Windows (Linux version uses GTK dropdown and therefore cannot be styled)
• Chrome/Linux

While i was at it, i added trivial theme switcher theme preview.

Closes #123 (closed)