This did not work. After reinstalling, setting the style to several different themes (and rebooting); then reverting back to Default Theme (and rebooting) the background colour of MS Excel spreadsheets remains grey (see image below). It's as if WB modified Office itself; or, changed the default theme itself. When I change the style the Excel background is modified to the new style colour. But after setting back to default, the default colour is grey (not white), based on the original WB installation.
I had installed Flat Dark and Dark Mode from the online repository. I notice they are still within the reinstall set of styles. Where are they stored? How do I delete custom styles?
Is there a way to edit the default theme in WB? One idea is that WB changed the default itself; perhaps it can be changed back using WB?
Or, perhaps there is some registry editing may be required to fully resolve this, which is of course dangerous.