Follow up to: #17619
Context: https://meta.discourse.org/t/introducing-discourse-splash-a-visual-preloader-displayed-while-site-assets-load/232003/17
We previously relied on the user's browser when deciding when to show the splash in light/dark mode. This worked well but can fail if the user manually selects a theme with a default "dark" scheme.
This PR will now factor that in. If the user selects a theme with a default dark scheme, use that. If a user selects a theme with a "light" default scheme and also picks a secondary "dark" scheme, use the media detection we had before.
This PR also removes the dark mode theme-color that was added in the previous PR. That will now go in a separate PR
3c558d2eb6