Fix #23508 : A custom style set as default style in Prefs. is not used

Fixed by loading the preferences before loading the custom style

Also makes sure the custom style is complete, by initializing it with the built-in defulat style before loading.
Merge pull request #550 from mgavioli/Fix_23508_load_default_custom_style

Fix #23508 : A custom style set as default style in Prefs. is not used