UX: Use DPageHeader on the Themes page (#30759)

This change updates the header of the admin Themes page to be more consistent with the rest of the admin UI.

app/assets/javascripts/admin/addon/templates/customize-themes.hbs

@@ -1,3 +1,18 @@
+<DPageHeader
+  @titleLabel={{i18n "admin.customize.theme.title"}}
+  @descriptionLabel={{i18n "admin.customize.theme.description"}}
+  @learnMoreUrl="https://meta.discourse.org/t/91966"
+  @hideTabs={{true}}
+>
+  <:breadcrumbs>
+    <DBreadcrumbsItem @path="/admin" @label={{i18n "admin_title"}} />
+    <DBreadcrumbsItem
+      @path="/admin/customize/themes"
+      @label={{i18n "admin.customize.theme.title"}}
+    />
+  </:breadcrumbs>
+</DPageHeader>
+
 <PluginOutlet @name="admin-customize-themes">
   {{#unless this.editingTheme}}
     <ThemesList

config/locales/client.en.yml

@@ -6043,6 +6043,7 @@ en:
           browse_themes: "Browse community themes"
           customize_desc: "Customize:"
           title: "Themes"
+          description: "Themes are expansive customizations that change multiple elements of the style of your forum design, and often also include additional front-end features."
           create: "Create"
           create_type: "Type"
           create_name: "Name"