i've not touched proboards in 5+ years with any serious intent of skinning, so take this with a grain of salt, but.

at bare minimum? it is completely and totally 100% possible to do a layout that has like. side-by-side boards like a table and if the last board doesn't have one to the right, it stretches across the whole thing. i've seen that done before and i don't remember how it was done but it is 100% possible. (i do remember it was a finicky nightmare thing to code back in like idk 2013, 2014, so probably not really the path you want to go down when we've found better ways to do it since then.)

but from what i remember of how proboards does the backend coding for the layouts, there's no reason why the same method jcink uses to do those custom layouts (explained here and here) wouldn't also work for proboards. my big "i might be lying" caveat is that i don't remember if proboards has an equivalent variable for that "forum id #" that makes the whole thing possible but it'd be a lil odd if it didn't so i don't think it's a lie.

but i'd also keep in mind that this only goes for v5, not for v6 (because shiva knows what's happening on v6), so y'know, not sure how easy or hard it'll be to execute the same basic principle after the software update nukes the code banks again.