Uh. You can.

In fact, the vast majority of desktop computer cases in the world are plain black boxes (or grey, or beige, but black seems most popular among the major manufacturers like Dell and HP these days). That’s because the majority of PC’s in the world are actually boring business boxes squatting on untold numbers of office desks. Just, no one posts pictures of those online because no one gives a shit about the 4,897 identical low-spec computers some megacorporation bought and had delivered on a pallet.

The PC’s you’re seeing are gaming rigs, and gamers festoon their stuff with silly LED’s because it’s cool, or at least they think it’s cool, or possibly just because it’s a meme by now. Or, in some cases, the piece of enthusiast gear you want comes with RGB LED’s all over it anyway, whether you like it or not.

You can absolutely build a gaming PC without the LED stuff, including buying a plain non-windowed case for it. I have a machine like that sitting on my desk right now, in fact.