Can you explain what you mean by "modify the corners"?
I think you can use the Site Builder to make any number of columns you like, and colors are all changeable.
I imported my site from Ning 2.0 almost a year ago. So I started with the standard jamroom Ningja skin (which I thought was somewhat unappealing but it was geared towards ex-ning users) and I began to change it to my own liking, saving it (using the 'clone skin' tool) as my custom skin with a new name. Now I have my own skin that I really like. I make changes as needed on my custom skin, but I leave a copy of the original Ningja skin intact and unchanged to apply JR updates to and then make 'compares' with my custom skin.
I imagine you too are going to want to make all the design choices on your own custom skin, and make it look just the way you want.

You can make almost any changes you want to YOUR custom skin- you're definitely not restricted to keeping it as it looks out of the box.
--
...just another satisfied Jamroom customer.
Migrated from Ning to Jamroom June 2015
updated by @strumelia: 04/14/16 09:56:30PM