Created attachment 9086 
In the upper part is the normal window list, in the lower part, unthemed window list

First, why I use bbkeys when XFCE has its own hotkey configuration. For two reasons: 1. bbkeys has some features that are absent from XFCE; 2. I prefer my hotkeys to be configured independently from DE, so that I can still use them if the need arises to load into some other DE or without DE at all, just with some window manager. Finally, it may be worth looking into because while I observe it with bbkeys, it may also be this way with some other applications.

Steps to reproduce:

1. Set up the window manager to display Alt-Tab as a list (Settings -> Window Manager Tweaks -> Cycling -> Cycle through windows in a list).

2. Press Alt-Tab. You can see the themed window list, about the same as in the upper half of the attached image (depending on your theme, of course).

3. Start bbkeys.

4. Press Alt-Tab. You can see the _un_themed window list, about the same as in the lower half of the attached image.

The theme used in the attached image is Vertex-Maia-Dark, but this glitch is observed with any theme.

As far as I can tell, this started after updating to 4.14, before it the window list called with Alt-Tab looked properly themed even with bbkeys loaded.