It helps if you use Shapeshifter to install the theme first (from the Apply section), then pull down the list of themes, pick the installed one, and click Tweak at the bottom of the window. A guiTweak window will open. Choose Colors, look at the list of items whose colours you can change and click on a colour patch to bring up a colour chart where you can alter the colour. The ones of interest are List Background and Sidebar Background.