Restrict custom style uses for free users

I've restricted access to custom styles for free users according to the details specified in the ticket.

Check before Pull Request is ready:

[ ] I have written a test and included it in this PR
[x] I have run all tests and they pass
[x] The code passes when running the PHP CodeSniffer
[x] Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
[x] Security and Sanitization requirements have been followed
[x] I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)