Is there any way to "overlay" the source/destingation window look? I'd love to make them more distinguished from other Opus' windows opened so if I'd make source i.e. more green and target more say red, that'd help a lot
You can change the colors in -> settings -> preferences -> display -> colors & fonts -> other colors -> file display border.
The first three items are the interesting ones.
Thanks. any hints why it does not work here? Do I need some other options enabled too (like aero related stuff)?
Sorry, i have no idea, why it doesn´t work for you. The aero settings shouldn´t affect the file display border graphics.
Anyone got that working then?
Without any screenshots or a better description than "it does not work here," it's hard to know what to suggest. 
The option Abr pointed out affects the colour of the File Display Border. If you're using a single-display window and have the border configured to only be shown when using a dual-display window, then it won't be visible at all. But in any other situation I can't think of a way that the option Abr suggested would not affect the colours of the border.
Ok, that's clearer now (by "border" I expected Windows' window border). I use single-display window and I would prefer option to change decoration of whole system window border/frame or even background. Anything that would make window visually different from others
Turning on some/all of these may help make it clearer which window is the source:
[ul][li]Preferences / Display / Options / Fade selected item colors when file display does not have focus
[/li]
[li]Preferences / Display / Options / Fade selected item colors when tree does not have focus
[/li]
[li]Preferences / File Displays / Border / Show file display border in single display mode[/li][/ul]