UI/themes: Fix missing icon in system theme

UI/data/themes/System.obt

@@ -179,6 +179,7 @@ OBSQTDisplay {
 
 OBSBasicSettings {
     qproperty-generalIcon: url(:settings/images/settings/general.svg);
+    qproperty-appearanceIcon: url(:settings/images/settings/appearance.svg);
     qproperty-streamIcon: url(:settings/images/settings/stream.svg);
     qproperty-outputIcon: url(:settings/images/settings/output.svg);
     qproperty-audioIcon: url(:settings/images/settings/audio.svg);