Hey Richard, And Welcome :)
Glad you like SS so far!. If you want to make a different color scheme you cant make a 'Subtheme' as such - they are designed for modules. But you could copy/paste the /themes/blackcandy folder to /themes/orangecandy or something :P. And then you would just need to change the site theme from blackcandy to orangecandy by opening the /mysite/_config.php file and editing the SSViewer::set_theme('orangecandy').
Once you have edited that your site will be using your own 'theme' so you can go ahead and tweak all the images, css etc under the /themes/orangecandy folder.
You can see some nice BC tweaks in the Site Showcase - this one was done by a GHOP Student and changed to a greeny teal - http://ghop-seanc.willr.co.nz/