tdf#164950 Set GtkCssProvider at APPLICATION level, not USER

- when set at the user level, the set_background() calls
  cannot override the theme settings as USER level has
  higher priority than APPLICATION level.

Change-Id: If8819b5838f6f9ff1cf5ec83e380b7546aafce93
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/180977
Tested-by: Jenkins
Reviewed-by: Sahil Gautam <sahil.gautam.extern@allotropia.de>

vcl/unx/gtk3/custom-theme.cxx

@@ -109,10 +109,11 @@ void ApplyCustomTheme(GdkScreen* pScreen, GtkCssProvider** pCustomThemeProvider)
 #if GTK_CHECK_VERSION(4, 0, 0)
         gtk_style_context_add_provider_for_display(pGdkDisplay,
                                                    GTK_STYLE_PROVIDER(*pCustomThemeProvider),
-                                                   GTK_STYLE_PROVIDER_PRIORITY_USER);
+                                                   GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
 #else
-        gtk_style_context_add_provider_for_screen(
-            pScreen, GTK_STYLE_PROVIDER(*pCustomThemeProvider), GTK_STYLE_PROVIDER_PRIORITY_USER);
+        gtk_style_context_add_provider_for_screen(pScreen,
+                                                  GTK_STYLE_PROVIDER(*pCustomThemeProvider),
+                                                  GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
 #endif
     }
 }