Bug 1955998 - Restore call to Theme::LookAndFeelChanged on init. r=dshin

This is not easy to test because this doesn't have any effect on
windows, and only startup is affected (so any ui.* pref that I could
tweak to write a reftest will "fix it").

I initially removed it to move it to nsXPLookAndFeel (thus making the
"refresh" and "init" code-paths more similar), but I think I forgot to
add it there, and it's not like it makes a big difference, so just put
it back where it was.

Differential Revision: https://phabricator.services.mozilla.com/D242746

Author: emilio

widget/Theme.cpp

@@ -168,6 +168,7 @@ void Theme::Init() {
   for (const auto& pref : kPrefs) {
     Preferences::RegisterCallback(PrefChangedCallback, pref);
   }
+  LookAndFeelChanged();
 }
 
 void Theme::Shutdown() {