Upgrading an office PC for gaming

I hope I'm not in the wrong community but I really wanted to get some opinions on that.

In short, it may be a it of an unpopular opinion, but I really hate the gaming aesthetics with all the RGB lights and glass and in general how modern 3rd party PC cases look like. On the other hand I really enjoy the look of a most of the recent Lenovo ThinkCentres and ThinkStations as well as Fujitsu's Esprimos. There is just something about this industrial matte black with red accents I can't resist.

What I would love to do is to take some cheap Lenovo Thinkcentre, slap a GPU in it and have a budget Linux gaming PC, but the problem is most of the parts in these office systems are proprietary. It seems that I can't easily upgrade the PSU to handle a proper GPU, nor can I swap the motherboard for a new one if I wanted to.

Does anyone have any idea on which models of ThinkCentres/Stations are easily upgradeable or of any cases that have this understated industrial office look to them? The only case I kinda like is the Fractal North, but I get really discouraged when I compare their prices to a fully equipped Lenovo office PC and it just doesn't look as good.

Also, I've looked into Lenovo Legion and HP Omen prebuilts, and while they're not as bad a 3rd party cases, I just don't like them as much as say a P520, which unfortunately comes with some awful proprietary motherboard supporting only Xeons from 2017.

Anyway, I'm sorry for the rant. I'd love to hear your opinions and suggestions.