In context menus I've seen only those horizontal '3d-style' separators. Is there any way to enable 'bitmap' separators in the [themename].wst file?
My guess is some value in SepType property but can't find out which one.

You can replace/modify SpacerA.png (or bmp) and SpacerI.png (or bmp) to your liking. Those are in the Winstep\Themes directory in the subfolder of the theme you use.

Thanks, the theme I was using simply didn't have those files so I had to create them.
Just for the records (in case someone gets into the same trouble than me), I also needed to change SepType=3 in [themename].wst, otherwise the separator is missing (SepType=0), flat (SepType=1) or 3D-style (SepType=2).
And I also needed to make sure that both the SpacerA.png and SpacerI.png files had the same height as ButtonVSize, in the [themename].wst as well, so the separator doesn't appear stretched (or compressed, depending on size).

Everything works as intended now, thanks