css not loading in secure browsers

Run a site report through 'Why No Padlock."  It may shed some light on your problem.

https://www.whynopadlock.com/

Let's see your page in question. If you are using an absolute reference to your file including the protocol then the protocol must have https. If the file is on a different server, that server must support the https protocol or you could end up with the error as well.