Yes, you can customize Auctiva templates. To see how to get to the customize page, hover your mouse over the listings tab and then click on 'customize' when the drop down appears.
Let me add that while you can do this, it is NOT for the fainthearted. In order to customize the template and not break it while you are doing so, you do need to be comfortable dealing with HTML coding.
I think the parameter you need to change to do what you said is "bgcolor" which you can find in the HTML code when you bring up the template in the template customization screen. We use 'hex' values so you will need to look up the hex value for the color you want.
Hope this helps.