Hello kishorat,
You can use Windows version of skin from Mac version. Just go to Applications folder and control+click on JRiver MC 20, and click Show Package Contents.
From there, double click on "Contents" > "Resources"
When you extract a JRiver MC for Windows skin file, it will have a Skin folder and may have a Data folder. Put whatever named skin folder into "Skins" folder and copy whatever inside of Data folder to "Data" folder of it. You will be able to see the skin name on the skins menu once you put the skin into "Skins" folder.
I hope this helps.
Best regards,
Y. G.