* gui, lib/api: Adds support for prefers-color-scheme on default theme (fixes#6115)
- Renames current default theme into a new "light" theme
- Modifies assets serving to allow getting assets from different themes
* lib/api: Serve assets from arbitrary theme when path starts with "theme-assets"
* lib/api: Moves constant out of function
* Loads light theme in browsers without support for prefers-color-scheme
* gui: Disables dark theme when printing
* Prevents repeated injection and adds support for older browsers
The CSS is always loaded if there is no support for `matchMedia`.
