Two or three years ago I tried using The Bat! but I remember that it was not possible to fully customize the colors of the user interface, in particular the background and foreground colors of the panes. For this reason alone I did not adopt The Bat! as my email client, although I was actually willing to do so (my problem is that my eyes get tired very much by the white background color, so I need a dark background).
Is it possible now to customize the colors of the user interface, especially the background color of the window? (Please, note that I am not asking about icons or themes, only colors)
Sure - OPTIONS / PREFERENCES / PROFILE LAYOUTS and one of the things that can be changed is the Background and also the search background
The wise would create a new profile first at least while you are learning. You can either hit the NEW button in the upper left or go one step up to Viewer Editor Profiles and do the basic creation there