Julie... To change the background... From the outer-most layer, if you want the changes to apply throughout...
Select Options > Editor > Change Appearance > Scroll down to Look and Feel...
Choose your new background by clicking on the blue arrows right/left
and then click on the small display to load the text into the text area below, as it says on the Alt Text applelet.
This works on mine... but I'm quite happy with the default... Hope it's OK for you!
Anne