I'm sure I missed something in the mod instructions. I did get it to do what I want though by changing line 1487 in the inc_func_common.asp file to the following:
CodeTags(1,1,2) = "<pre id=""code""><font face=""courier"" style=""background-color: yellow"" size=""" & strDefaultFontSize & """ id=""code"">"
This makes the code background yellow.