Thank you! That solved it--I added it in the Quick CSS section.
However, I still can not find this custom.css file and I'd really like to for other tasks. I'm sorry--this probably seems so basic, but my admin nav does not seem to match what you map. I can not find wp-content or a CSS folder. I've looked through everything to see if I can find this and I can't. Am I on a different wordpress version? Do I lack administrative access?
Thanks for your help.
These are my nav choices: