Now that we have support for user-selectable color schemes, it makes sense
to simplify seeding and theme updates in the wizard.
We now:
- seed only one theme, named "Default" (previously "Light")
- seed a user-selectable Dark color scheme
- rename the "Themes" wizard step to "Colors"
- update the default theme's color scheme if a default is set
(a new theme is created if there is no default)