Simplify theme and color scheme seeding (PR #10872)

Now that we have support for user-selectable color schemes, it makes sense to simplify seeding and color scheme updating via the wizard.

This PR does the following:

  • seeds only one theme, named “Default” (previously “Light”)
  • seeds a user-selectable Dark color scheme
  • renames the “Themes” wizard step to “Colors”
  • in the wizard, updates the default theme’s color scheme if a default is set (a new theme is created if there is no default)