I’m glad you’re enjoying the theme. You can change the color and transparency of the sidebar. The background color uses what’s called a RGBA color (red, green, blue, alpha). The last number is the “alpha” value that sets the transparency of the color. It uses a scale of 0.0 (completely transparent) to 1.0 (completely opaque). There is a color tool at http://www.css3maker.com/css-3-rgba.html that can help you find the right value. Here’s how to use it:
Go to your theme options page and choose the Advanced tab.