It's definitely related to the Tree_Selection.png file (along with retina mode + skin size/scaling), because once I add back the file to the Modern Cards: Dark Edition's folder and reload the skin, I can see the white lines again.
That said, I added marko's Tree_Selection.png from his modified Modern Cards: Dark Edition skin (located
here) to the skin, it seems to completely fix it. If anyone else confirms this fixes it for them, looks like this issue can be easily fixed, at least for the Modern Cards: Dark Edition skin.
I've attached marko's Tree_Selection.png file for you guys to try.
EDIT: Believe it or not, this issue also happens in the list (e.g. Artists/Albums list in panes view) as well, it's very hard to notice though. Seeing if marko's changes in List_Selection.png fixes that too.
EDIT 2: Yep, looks like marko's List_Selection.png fixes that issue too. Okay, so this issue exists with both
List_Selection.png and
Tree_Selection.png so I've attached marko's List_Selection.png to this post for you guys to try too.
EDIT 3: Looking at tyler69's GIF above, it also happens with the stoplight button images, which I think is a similar but a little bit different issue that's related to skin size/scaling with retina mode enabled. So that's something else I'll have to see if I can reproduce since that most definitely looks like a skin issue too. What you might try since it looks like you're using around 100% scaling is to go up one level in skin size/scaling (e.g. from 100% to 110%) to see if that changes anything. If so, it's most definitely related to the skin and using retina mode + scaling. So, tyler69 what's your screen resolution and what is your MC skin size setting set to? Also does disabling retina mode 'fix' it?
For the other main issue, going to need some confirmation on this and if the two fixes work for you guys. If so, the devs can probably import marko's changed files into the main skin and call that one fixed.