Support nav themes in dark mode

Beta dark theme overrides navbar, we should bring the colorful nav back!

I think 'Theme' is the most widely used name for this. Went with 'Color scheme' in the mock so it isn't confused with Nav themes.

Example cobbled together in devtools (using syntax highlighting for label images)

Would also be good to have "automatic", where you can choose preferred theme for light/dark. So if you wanted dark mode during the day and light mode at night. This will be more useful when we add low and high contrast modes.

Should we limit allowed nav themes based on light/dark selection? There are likely some incompatible options, contrast-wise. Making them all work is possible, but might be excessive effort