restore $default-foreground and $default-background vars

[vpetrusevici]

Description

Hi, guys! I just noticed that some of deleted foreground and background vars are still used in library logic. So after deleting them service just sets unusigned css vars. I think we should restore them for now. Describe what this change achieves

Issues Resolved

List any existing issues this PR resolves

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented in the README if applicable.

[johannesjo]

Thank you!