Replace last remaining use of `h-branding` with `applyTheme`

src/sidebar/components/hypothesis-app.js

@@ -5,6 +5,7 @@ import uiConstants from '../ui-constants';
 import { parseAccountID } from '../util/account-id';
 import isSidebar from '../util/is-sidebar';
 import { shouldAutoDisplayTutorial } from '../util/session';
+import { applyTheme } from '../util/theme';
 
 /**
  * Return the user's authentication status from their profile.
@@ -57,6 +58,8 @@ function HypothesisAppController(
   // used by templates to show an intermediate or loading state.
   this.auth = { status: 'unknown' };
 
+  this.backgroundStyle = applyTheme(['appBackgroundColor'], settings);
+
   // Check to see if we're in the sidebar, or on a standalone page such as
   // the stream page or an individual annotation page.
   this.isSidebar = isSidebar();

src/sidebar/templates/hypothesis-app.html

-<div class="app-content-wrapper js-thread-list-scroll-root" h-branding="appBackgroundColor">
+<div class="app-content-wrapper js-thread-list-scroll-root" ng-style="vm.backgroundStyle">
   <top-bar
     auth="vm.auth"
     on-login="vm.login()"