Originally Posted by PhilJ
There's two images - tab.png and tab_current.png
The source file is included for both.
To change the right hand side button colour, it's just one line of CSS.
Oh, I see.
Just my 2 cents, but as it seems that you are specifically targeting people with no CSS knowledge, it may be a good idea to simply include the css line in the settings page. Granted, they would still have to use a site like http://www.w3schools.com/tags/ref_colorpicker.asp
to enter it into the settings page, but that would be one less step.
Also, since it would not take much time to make, you may want to include 10 or so pre-made color buttons (blue, green, etc).
Even if you do not include my suggestions, I still think you have a quality product. Great work.