The treebar's background is wrong. The treebar's border is changed and the background is not changed. So does the command /backgroud -b

This is not a bug. It is how the treebar is designed. Apparently, the amount of updates needing to be done constantly to the treebar makes a full background too slow, so it's not allowed.

Allowing a single color should be allowed....the white is burning my eyes against the rest of my black background windows.

The treebar's background colour can be changed in the "mIRC Colors" dialog (Alt+K).