Need help with customization

Im currently working on a lister theme which simulates ACDSee Pro 2. Up to now it looks like this:

Is there a way to override window border colors which separate the folder structure and the two listers to be black or dark grey ? Or can the width of them be reduced to say 1 or 2 pixels ?

Thx for any hint.

The splitter bars should be using whatever colours are configured for your system, i.e. in the Display/Appearance control panel.

There isn't a way to override them just for Opus and not the rest of the computer.

Ok thanks