I only keep 2 max 3rd party themes on my berry, also dont leave viigo open in the background, it checks and updates, I only open it when I want to use it.
I don't think it had anything to do with the themes, they actually do not take up as much memory as people believe. I think you might not have been closing your applications out properly by hit menu button>scroll down to close. Don't escape out of an application by pressing the escape or end call key. If you leave Viigo running in the backround it will use up alot of memory as it is constantly updating along with what other apps you got...