I've searched but I didn't find anything...

How do you add lines in colums ui?
I would want to have lines between my albums so i guess it's something with tracknumber one... but how do i do that?

you can define colour like this in Globals tab - Colour, or Columns tab - Colour.

text|selected text|
background|selected background|[selected background non-focus]|
[left frame]|[top frame]|[right frame]|[bottom frame]

example:

Thank you!