Most of the topics you find here in
the Theme board are publicly shared theme’s so you could take a look there.
As for as making your own, if you know HTML and CSS you’ll be good. GetSimple takes pride in being simple, so a theme is essentially a normal HTML file containing just a few PHP functions for things as displaying content. These function can be
found in the GetSimple documentation.
Changing the credit note is a little more tricky. This depends completely on the license the theme is under. As most themes you can download here have not specified any license they are automatically under standard copyright law and taking away the theme maker’s name (if it is shown at the bottom) would be copyright infringement.
The standard themes are licensed together with GetSimple under GPL2. This would mean you have to license your edits under the same license. I do believe though that this means you don’t have to visually credit Chris with the creation of it as long as it is specified somewhere and the GPL2 license is included. I’m not sure though, I didn’t take the time to dig up the right clause.
A link back to GetSimple in the footer is never required, so that one can be taken away.