Using the same theme on two sites
I'm the proud owner of a shiny new 'StudioPress Pro Plus All-Theme Package'.
I'd like to use the same theme on two different sites, in the same Wordpress network installation (directory, not sub-domain).
How do I go about this and how can I customise each site without it affecting the other (e.g. in Lifestyle, say I wanted to change the header and/or colours in the CSS, for one of the sites only).
I may want to 'share' a couple of the themes I have purchased, so suspect this may be a generic question.
Thanks in advance