There is a very thin horizontal line with group columns enabled. It doesn't look like it's there on purpose, as it does with the standard group header/divider. It doesn't look very neat, especially when using a background image.
Is there a setting somewhere that i missed?
Directory Opus Pro 12.22 Build 7593 x64 OS 10.0 (B:19042 P:2 T:1) SP 0.0
That's normal.
Set the Frame color to the same as your background color under Preferences / Display / Colors and Fonts / File group column.
The frame is set to transparent. 
Setting a solid color globally doesn't work for me, as not all folders have the same background color, and I use an image as backdrop in others.
Transparent in this case means it will use the fill color instead of a separate color.
There is supposed to be a line between groups, so that you can see where one group ends and the next begins.
Respectfully. The groups are visually divided by their label more than any line does. It's not strictly necessary in my opinion, and moreover it does not add any value being extended beyond the last column, or even the other columns.
Ideally, the border would continue the negative space between items and not add anything, as i would expect from being transparent. That would visually be the most elegant for sure.
There's nothing to be done then? Because it really ruins the look of group columns for me.
It's working as designed. We might make the colors/parts more configurable in the future, but not in the near future as we need to finish other work first (including an overhaul of the colors system to make it easier to add more colors settings for individual details like that line).
Thanks Leo, that would be very welcome.
It is definitely hit and miss with the color customization at the moment.
More hit than any other piece of software I can think of. You're usually lucky to get a choice of two themes. 
Indeed. But more options also means the details that start to bother you get smaller and smaller.
