CSS Editing Issue

Hi,

I have my multisite network running well with the theme i wish to use installed on the network.

My issue is although i would like to use the same theme across all the blogs, i wish to make changes to the css (change some colours, add some images etc..) so it would be slightly different for each blog.

Although my theme allows me to change a lot anyway it is just a few things that need changing in the css, which are causing me the hassle.

Is there a piece of code or a plugin which will allow me to do this quickly?

Otherwise im looking at having to install many variations of the same theme?

Thanks