Handling of theme options for the navigation redesign Beta

Details

Since we will only support Light and Dark Mode themes for the Beta, we should indicate to this to users on the "Color theme" preferences.

Proposal

Add a note to the description of this setting when Beta is turned on to inform the user only Light and Dark Mode have effect for that experience.

Note: You have the new navigation enabled, so only Light Gray and Dark Mode themes have any visible effect.

We just need to conditionally render some additional text here based on the show_super_sidebar? helper. Though, we probably want to skip the super_sidebar_supported? check, since the preferences page doesn't actually support the sidebar yet.