Colors issue

Hi there! I’ve recently started using the Rehub theme for my projects (This one, for example, ecbooks.ca) and people start complaining about color issues on different tabs & pages. It happens randomly, as far as I understood. But mostly with Firefox users and those who use MacOS.
I’m not a Mac user, so can’t check the problem.
Does anyone have the same experience or should I contact the author directly?

That could be any number of things -

Presumably if it’s multiple projects/websites then you have a unique license/copy for each?

Are you definitely using the latest copy of WP, the theme and hosting php?

Assuming you are within your support period then you are probably best to contact the author but I would try and get screenshots (ask someone who is seeing the issue or has a Mac) etc first to demonstrate the issue