Alternative CSS tags to “display:none;” for SEO [duplicate]

I have always made my pages "by hand". just coding them, a bit of javascript here and there.

And because I like to experiment with designs, i use a lot of popup divs, which means i use "display:none" tags for those divs.

I’ve been told that this could be hurting my searchability. The pages have been online for a long time. does anyone have some advice on this?

Someone has been mentioned that I will have difficulty with SEO and that I need to use a CMS like wordpress.

what do you think my next move should be?

Is there something in my CSS I can change, or do I go the CMS route?