Using one theme with different css? Best way to do it?

[kerugma]

Hi,
I'm using Serenity theme on my site. I have also set up a multisite network to allow 3 other sub directories. (My terminology might not be correct).

I have renamed my original serenity them and it is installed on the main site. I would like to change the colour scheme in Serenity and use it on the other 3 sites.

I'm a little unsure as to the best way to go about this. Do I use a plugin or do I rename the theme with 3 other names and then upload them?

Thanks for reading,

Susan