Can i use the existing theme made by user and redesign it. I know i should get proper permission from the author. Apart from this should i do anything?
Modification for personal use should be fine, I think, but to be published and shared as a new theme, I think you should have the author permission.
Most themes are only css variant from default so don't worry that much.
Just like
@ahrasis stated. I'll add that if its one of the ones I did and published on this site, then have at it :D