Hi there - Brave has the Light/Dark theme, as you’ve found: brave://settings/appearance
As of Chromium 76 websites can query if you’re using dark mode:
When I visit sheets or Google docs with dark mode enabled, things look OK (it seems to be light mode in Google docs). You’ve already shared you don’t have any extensions like Google Docs Dark Mode installed, so that’s good:
What happens if you change the browser back to to Light mode? Does Google Docs / slides pick that up?