Add label for admin theme selector (mastodon#11121)

* Add simple_form default for admin theme selector

* Revert "Add simple_form default for admin theme selector"

This reverts commit 0b736f7.

* Add setting_theme label to admin theme selector

app/views/admin/settings/edit.html.haml

@@ -9,7 +9,7 @@
 
   .fields-row
     .fields-row__column.fields-row__column-6.fields-group
-      = f.input :theme, collection: Themes.instance.names, label_method: lambda { |theme| I18n.t("themes.#{theme}", default: theme) }, wrapper: :with_label, include_blank: false
+      = f.input :theme, collection: Themes.instance.names, label: t('simple_form.labels.defaults.setting_theme'), label_method: lambda { |theme| I18n.t("themes.#{theme}", default: theme) }, wrapper: :with_label, include_blank: false
     .fields-row__column.fields-row__column-6.fields-group
       = f.input :registrations_mode, collection: %w(open approved none), wrapper: :with_label, label: t('admin.settings.registrations_mode.title'), include_blank: false, label_method: lambda { |mode| I18n.t("admin.settings.registrations_mode.modes.#{mode}") }