In case anyone else finds this small issue, Grimpel notified me:
Found another minor display bug in admin_trees_config.php:
On my system tab “Theme” doesn’t display the F.A.B.-screenshot.
The page is calling screenshot_F.A.B..png but the file is named screenshot_f.a.b..png
I renamed the file as screenshot_F.A.B..png and everything is fine.
As reported, it is fixed (ready for next release) by simply changing the file name to use the upper-case form as the theme name itself.