Improve theme page

[techniq]

• [ ] Remember custom theme colors if you navigate away and back (store/read from localStorage, directly from stylesheet, ...)
• [ ] Fix parsing of hsl(200 100% X) (typing X causes it to be parsed as hsl(0 0% 100%)
  • hsl(200, 100%, X) works and becomes hsl(200 100% 50%) once you type the last character ())
• [ ] Handle using a dark theme as the only theme
  • Add toggle for single or light/dark
  • Add all light and dark option in a single selector
• [ ] Applying custom themes as overrides should also set theme selector to light / dark only
• [ ] Show more preview examples than just the palette (show Buttons, cards, etc)
  • Might not be as needed once you can override the docs page, but still helpful to have "at a glance" preview
• [ ] Supporting loading a themes.json file, or copy/paste JSON? Might be difficult to handle a lot of customization