Quote - ... and I can't figure out which (if any) of the css codes refer to this icon.

This is done with two different images not CSS.

You could get control over the background colour of the relevant table cell if the site code was changed to generate a different style class for cells that use each image.

I suspect that that's the best you could hope for as it involves the least modification and only uses CSS, all the other solutions would require additional images and a way for you to pick them.