I wouldn't say it's very difficult, but it's hard, yeah. It's hard for three reasons (from my perspective):

(1) There's a lot of subtlety to the current HTML/CSS. That is, in a lot of places, it works well, but it's kind of ossified into this fragile mass of markup and styling interdependencies. For example, it took me ages to figure out exactly the right way to fix this (almost every small adjustment I made ending up breaking the layout in some subtle and sometimes very difficult-to-reproduce way; in the end, I found the right spell/incantation, but, damn, it wasn't easy). So, I expect a serious effort to modernize SMF's markup/styling to be a much larger undertaking than it might initially appear.

(2) As a (kind of) corollary to the previous point: there's also a history-preservation/conservator lens to look at this issue through. I think the forum has a very important legacy, and it shouldn't be messed with by just anybody. It's kind of like if you wanted to have a very important artwork restored: you'd look for someone that seriously knows what they're doing; it would be a mistake to let the wrong person anywhere near it. Anybody that says: "Sure, I can do that, it's easy!" is going to mess it up, mark my words. (I mean, "mess it up" is different for different people, and I've witnessed what I consider to be a total fuckup go on to be praised by others, so, I'm not speaking in absolutes, I'm just saying that the end result wouldn't pass muster with me.)

(3) There's a way to completely avoid the previous two hazards, by doing the visual overhaul within the context of an alternative frontend. That way, SMF is left (just about) as-is, with only minor changes to it every once in a while, but there'd be another frontend where we could basically go nuts and try all sorts of fun stuff (without worrying about either of my previous two points). In fact, I'm pretty certain that I could make it so that switching (in either direction) between the legacy frontend and the modern one would be as simple as clicking a button. The problem with that approach is that there's a large amount of technical groundwork to be laid before it would become feasible (and, though I'm willing to do that work myself, it currently falls outside my purview).

In summary: It's harder than it looks, it's probably not a good idea to begin with, and the work necessary to make it easy and sensible hasn't been done yet.

P.S. Don't be discouraged by my answer, the work you did is really cool, it's just that, when looked at from many angles, it's a very small part of the puzzle.