Hello Team,
This looks like a bug. I have a Cards region with a Cards Container template. This is a simple ICON-only card using FA Icons (as a column). When using the “Redwood Light” theme, it all looks great. Once switching to any other theme, the entire card(s) looks really bad.
Environment
APEX on OCI
(Reproducible on apex.oracle.com)
Demo: https://apex.oracle.com/pls/apex/r/lktg/apex-issues/theme-changes?session=26910469800683
After running, change the theme 
at the bottom.
Example:
Switching theme (Vita/Dark/…)
For @apex-developer, I have a sample app that I can send by email.
Thanks,
Lior