This allows to set the background color for Pygments text.

This was originally added in 4328974b86f10ef4a7e828d647cd04d5c3a2f33e
then reverted in 0736cf338ae810d9b4e374360589c6d7ada229cf. Here is the
reason:

https://github.com/cyrus-and/gdb-dashboard/issues/116#issuecomment-386554430

Close #252