It would not be difficult at all because the guests don't actually see
Which is why it would be difficult. You'd have to give them that ability as a first step
Actually, the real problem with tying parts to themes is what we see already when looking for specific parts with the theme filter. Parts that should be in 1 theme are arbitrarily assigned to another. Even worse, the theme sets don't all have corresponding parts, so either you'd need to invest a bunch of time into making a bunch of redundant parts differing only in their textures and theme tags, or you'd have to give most parts tags for many if not all of the themes. Which defeats the entire purpose of what you're trying to do here, because the result would be that no matter what parts you used, you'd still get the theme bonus.
It would be far easier to have 1 small little easily hidden part for each theme. This part would have the effect of saying "everything within X radius of me belongs to Theme Y". Then you wouldn't need theme tags on the parts, and could use whatever parts you wanted in the nearby scenery. BUT, OTOH, if that's all you have to do to get the bonus, then everybody would and there'd be no real point in having the bonus.
And finally, there's the problem that if you have bonuses for game-defined themes, you limit the creative ability of players to invent their own themes. For example, I tend to make many buildings in the same park or area out of the same mixes of a couple of stock themes to create my own theme that the game doesn't give me. For example, Spanish and French tropical colonial. This uses modern (stucco, brick, wood, and concrete) plus western (lime plaster), all covered with triopical/pirate bushes. At present, I'm neither helped nor hurt by this, yet my park has a consistent appearance which I think realistically captures a specific historical architectural style . However, if theme bonus points come into play, then I'd be hurt by doing this. My buildings wouldn't be pure-blooded in any specific theme and my building themes wouldn't match my gardening theme. I'd be rather miffed by that
So really, there isn't a good way to implement theme bonuses. No matter how the devs might try to set it up, it's either going to do more harm than good, or result in a bunch of part redundancy, or computer overhead, or all of the above, for what is objectively essently ZERO gameplay effect. Which is no doubt why the devs very sensibly opted not to have this antiquated and misguided feature in PC, thus allowing much more player creativity. Make no mistake. Theme bonuses are simply limits on what players can do. That's a bad thing. Not having them is freedom. That's a good thing.