There is no light background built in. To use a custom white background:
Create your own color them in Prefs->PCB editor->colors
There’s a dropdown to make a new theme. That way it will all be saved.
You can change the background color in the menu on the same prefs page. So so far so good.
The color algorithm however - just lightens the color when selected - so you probably want kind of light gray - rather than white - background. That way selected colors that are too light will still kind of show up. This will take a bit of experimenting and likely be monitor specific
I am currently using my own light theme and it’s working well enough but the color algorithm could be much smarter - and that is occasionally makes things tedious. I’ll be happy to share it if you care - I’m not quite sure how - if you can message me here or what.