Simplify theme and color scheme seeding (#10872)

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)

lib/wizard/builder.rb

@@ -181,26 +181,22 @@ class Wizard
           next unless scheme_name.present? && ColorScheme.is_base?(scheme_name)
 
           name = I18n.t("color_schemes.#{scheme_name.downcase.gsub(' ', '_')}_theme_name")
-          theme = nil
+
           scheme = ColorScheme.find_by(base_scheme_id: scheme_name, via_wizard: true)
-          is_light_theme = (scheme_name == ColorScheme::LIGHT_THEME_ID)
           scheme ||= ColorScheme.create_from_base(name: name, via_wizard: true, base_scheme_id: scheme_name)
 
-          themes = Theme.where(color_scheme_id: scheme.id).order(:id).to_a
-          if is_light_theme
-            themes = (themes || []).concat(Theme.where(color_scheme_id: nil).order(:id).to_a)
-            themes.sort_by(&:id)
+          if default_theme
+            default_theme.color_scheme_id = scheme.id
+            default_theme.save!
+          else
+            theme = Theme.create!(
+              name: name,
+              user_id: @wizard.user.id,
+              color_scheme_id: scheme.id
+            )
+
+            theme.set_default!
           end
-          theme = themes.find(&:default?)
-          theme ||= themes.first
-
-          theme ||= Theme.create!(
-            name: name,
-            user_id: @wizard.user.id,
-            color_scheme_id: scheme.id
-          )
-
-          theme.set_default!
         end
       end