Bug 1619664 - Remove nsBox::gTheme. r=dholbert

The boxes can just poke at the prescontext theme like literally everything else.

Differential Revision: https://phabricator.services.mozilla.com/D65161

layout/xul/nsImageBoxFrame.cpp

@@ -606,9 +606,12 @@ bool nsImageBoxFrame::CanOptimizeToImageLayer() {
 
 imgRequestProxy* nsImageBoxFrame::GetRequestFromStyle() {
   const nsStyleDisplay* disp = StyleDisplay();
-  if (disp->HasAppearance() && nsBox::gTheme &&
-      nsBox::gTheme->ThemeSupportsWidget(nullptr, this, disp->mAppearance)) {
-    return nullptr;
+  if (disp->HasAppearance()) {
+    nsPresContext* pc = PresContext();
+    nsITheme* theme = pc->GetTheme();
+    if (theme && theme->ThemeSupportsWidget(pc, this, disp->mAppearance)) {
+      return nullptr;
+    }
   }
 
   return StyleList()->GetListStyleImage();