Adjusting theme background size and bookmark highlight brightness

Is there some way to adjust the default size of the background picture from a theme?

My desktop resolution in 1024x768.

When I add a new theme it installs just fine. However, the background image is too big and a lot of the image is lost.

I noticed that some of the images have info in the description for blogs or other sites where you can obtain the theme in additional sizes. However, most of the themes do not have this option.

I have also noticed another problem. I switched to a new flat-screen monitor and since doing so I have noticed that the highlight you get when moving the cursor over bookmark items is extremely light. I tried adjusting the monitor thinking it was at fault, but no adjustment would make it any darker.

I take it that there is nothing I can do about this? I was hoping changing the theme would fix it but it did nothing to help whatsoever.