My toilet is too far from the wall. There is almost a 2 1/2" gap between the back of the cistern and the wall. The toilet is on a marble floor and there is a marble baseboard that is about 1" thick. The toilet I currently have has a 12" rough-in (the space from the center of the bolts to the wall, right?) but the drain is, I think, farther from the wall than that. The measurement from the center of the bolts to the marble baseboard is about 14". Is there an adaptor that can bring the toilet closer to the wall? My bathroom is narrow, so any additional space is good, plus it looks odd to have such a large space between the toilet and wall. I am replacing the toilet with a new toilet with a 12" rough-in.

You can buy an offset flange that will move the commode back about 1 1/2 inches. It will depend on what kind of pipe is used in your plumbing as to what kind of flange you will buy. You will have to chip some tile and some concrete to install the new flange, but all of this should be hidden by the commode base.
At one time you could buy a commode that was far a 14 inch rough-in, but I have not bought one in years and I am sure if you did find one it would be expensive.. Good Luck.

Letme is correct about the offset, might even be 2 1/2". The 14" rough in toilets are readily available but not at Home Depot or the like. They can order them for no additional charge.