From the demo forums you just need to take older forums style CSS
(right mouse click on the link - Save Target as..., it's called by us the "Integrity skin"), copy it to your miniBB folder, apply it modifying $skin='old'
under setup_options.php, and under the same file set $startPageModern=FALSE;
You will need to modify CSS to make colors matching your website.
Then download "classic" forums header file in the old mini style
and replace it to the current one (located under templates/main_header.html folder). Of course, later you will need to adjust it a bit to your own forums, maybe putting forums logo
, modifying other HTML parts etc. It's your own task.
This will fully revert you to the older layout.