nixosTests.lomiri-clock-app: Fix OCR further

Localised strings still struggle to be found consistently. Issue seems to be that the app has a somewhat small
horizontal max size, leaving more than half of the screen at the same colour as the app's text. Set IceWM's
background colour to #FFFFFF to fix that discrepancy in background colours.

nixos/tests/lomiri-clock-app.nix

@@ -18,6 +18,12 @@
         variables = {
           UITK_ICON_THEME = "suru";
         };
+
+        # App has a somewhat small horizontal max size and a white background, while we configure IceWM to have a black background.
+        # Makes OCR less reliable, often completely fails to find the localised text. Force background to be white instead.
+        etc."icewm/prefoverride".text = ''
+          DesktopBackgroundColor=#FFFFFF
+        '';
       };
 
       i18n.supportedLocales = [ "all" ];