Thanks for the response, Marco.
Unfortunately, renaming the registry key did not resolve the issue :(
Unless there are any other suggestions/ideas, I will likely recommend to my co-worker that she should have IT uninstall and reinstall PL/SQL Developer.
Thanks again for your help!
A co-worker is using version 5.0.3.527 and her syntax highlighting stopped working even though "Syntax Highlighting" is checked off in the Editor preferences. Apparently, syntax highlighting stopped working after she clicked something in PL/SQL Developer... unfortunately, she doesn't remember...