[FEATURE]: Developer guidelines to prevent light-mode visibility issues in the future

[koichincom]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I've PR'd some light-mode visibility bug fixes (#5352 and #5378) that mainly stemmed from the lack of fg={theme.x} properties, as you can see in this diff.

It would be great if this could be documented in the developer guide or agents.md so it won't be forgotten in the future and the bug won't recur, since it's tough to track and new text elements are added frequently.

[rekram1-node]

yeah good point