GD-1061: Improve readability on light themes

[charjr]

The used GdUnit4 version

6.0.3 (Latest Release)

Godot Version

4.5.1 stable

Environment Details

Linux (Debian)

Bug Description

Using light backgrounds makes several elements unreadable.

Steps to Reproduce

Editors Settings > Interface > Theme > Preset: Light

Minimal Reproduction Project

No response

[MikeSchulze]

Yes, I've been putting off the topic of themes for a long time :)
Some of the colors are hard-coded, as they are not currently mission-critical and are tentatively scheduled for version 6.2.