Options for changing the background colour can be changed via the 'View' menu and then selecting 'Background Colour'. The Colour can be changed by clicking on the square next to the word 'colour'. There are a range of pre-defined colours but custom colours can also be created.
Moving the horizontal slider (within the 'Panel Transparency' section) furthest right, will make the entire interface completely opaque. Likewise, sliding fully left will render all but text and essential interface options, fully transparent.