Respect and detect prefers-color-scheme: no-preference

[emilk]

I don't think this will make a difference in practice, but technically there are three preference states:

• dark
• light
• no-preference

Previously we would only check for dark, and if not set would assume light.
Not we also check light and if we're neither dark or light we assume nothing.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7293-emilkno-theme-preference
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.