I don't know.
As a Links user (no javascript), the worst problem I see with modern forums is that a lot tend to go from mostly pure HTML templates, which simply work everywhere if you care to tweak your CSSs, to do more and more stuff through JS (e.g. vbulletin has replaced simple links to paged threads or search results with some JS BS).
A "modern" browser where JS fails regularly can be even worse, so you probably want to choose a forum that does not rely on that for basic functionality, and the rest is a matter of editing HTML and CSS, provided the forum software lets you do it. As I said, SMF used to be pretty good in that regard, maybe it still is.