mutliple themes on same website

Hi!
I know there is at least a plugin which does that, but I’d like to do it on my own. I would like to have different themes (3 of them) on different pages/sections of my website.
I’ve worked with templates already, and I’ve done a website where each category had its own css, but the main website was the same (https://www.espacescomprises.com The main layout is the same, but each category has different colors).
Here, I want the general layout to be different, so I’ll need to call a different header and a different footer. My main concern is: how do I do that? If I do some conditional statement with the loop, like “if this page, then load this or that”, it means I have to wait for the loop. What do I do with the header call? Or is it any other way to do it?
Example:
If I need a specific header and footer and css for the contact page, I’ll have to use the “is_page( ‘contact’ ) ” and then load the stuff I need. But that query comes after the get_header() call since it does inside the loop, doesn’t it?
How should I code it so that I don’t risk calling the header once it’s already been called?
Thanks!