Fix custom theme not found when compiled with QRC

If the application was compiled with QRC (i.e. CONFIG+=dev), a theme
setting that wasn't one of the default theme would fail to be read.

Originally commit cf463f24786566c5ec04a6823e9917975ba2b265,
accidentally thrown out before release

src/backend/user_files.py

@@ -519,7 +519,10 @@ class Theme(UserDataFile):
 
     @property
     def default_data(self) -> str:
-        path = f"src/themes/{self.filename}"
+        if self.filename in ("Midnight.qpl", "Glass.qpl"):
+            path = f"src/themes/{self.filename}"
+        else:
+            path = "src/themes/Midnight.qpl"
 
         try:
             byte_content = pyotherside.qrc_get_file_contents(path)