I’m interested in this too. This guide shows how to add a header: Best way to customize the header
I think the tricky bit (for beginner me) is getting the header out of WordPress so that Discourse has everything it needs to make it look the same. This is perhaps more complicated by the fact I use Avada for my WordPress site.
---- Update ----
Have done a bit more digging. In theory would I just need to copy the CSS theme and ‘header.php’ html into the appropriate places in discourse? or is it more complex than this … .
— Last update before bed —
So that doesn’t seem to work. Discourse doesn’t like it when ‘php’ is in a line of html. Not sure why.