Can I go directly from 3 to the current version of 5 or do I have to go to 4 first?
You have customized css files for a top nav for us at www.glss.website. We are upgrading to get the accessibility features in 5. Will this totally blow up the custom css (even if we reinstall the custom css?) or will the custom css totally blow the new accessibility features of Porto 5?
You can directly upgrade to Porto 5.
It would require customization to make your Porto 3 theme compatible with Porto 5. Please open a support ticket https://www.mandeeps.com/support so that we can provide you the estimate Time Slots.