Hi there Roller. I setup MC on Majaro about 2 months ago and have been proceeding to migrate across. I doubt it is different between distributions as MC follows the convention of creating .JRiver folder under your home user folder. I copied my own skin folder directly under the 'Skins, Standard View' folder in this location to allow it to be selected.
/home/<user>/.jriver/Media Center 30/Skins/Standard View
I copied a skin folder which is a mash up of Modern Dark but the background images are photos of Vinyl black LPs - so very easy to see it's mine. I created this by mashing together aspects of 2 other skins. I have been reusing this folder in every version of MC for a while. I would struggle to remeber what I did in terms of the files themselves. I closed MC, added the folder and files and restarted MC. The skin was available as one of the menu options. There is no reason why the Theatre would not behave the same, but I have not added one there.
I think I am only a few weeks of where you are but I came from Windows so had drive letters and /\ to contend with. I managed to get my playlist across as part of the library export import process.
David