[Global] Update refs to UI.themeSupport

Bug: 1006759
Change-Id: I8d11fd8de1f43e2add97dd09dd539bfda4a23f39
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2019560
Commit-Queue: Paul Lewis <[email protected]>
Reviewed-by: Tim van der Lippe <[email protected]>
diff --git a/front_end/ui/utils/append-style.js b/front_end/ui/utils/append-style.js
index 385bee9..d9d2d29 100644
--- a/front_end/ui/utils/append-style.js
+++ b/front_end/ui/utils/append-style.js
@@ -16,7 +16,7 @@
   styleElement.textContent = content;
   node.appendChild(styleElement);
 
-  const themeStyleSheet = UI.themeSupport.themeStyleSheet(cssFile, content);
+  const themeStyleSheet = self.UI.themeSupport.themeStyleSheet(cssFile, content);
   if (themeStyleSheet) {
     styleElement = createElement('style');
     styleElement.textContent = themeStyleSheet + '\n' + Root.Runtime.resolveSourceURL(cssFile + '.theme');