A few things:

• Pretty much all PC prebuilt companies favour glass and RGB, in part because RGB is popular, in part because it makes the system look more expensive, and can help feel like it justifies the costs.
• Tempered glass, as is used in PC cases, tends to shatter into cubic-ish fragments, not into knife-like shards, like you might be used to with most glass or ceramic items. Having to pick glass shards out of your skin because your PC case broke is extremely unlikely, even if it broke on you.
• While cases with no windows absolutely exist on the prebuilt market, they generally still have RGB. For example, Dell/Alienware systems.
• Some cases have two options for the side panel (often along the lines of perforated vs glass) and, in general, if that's the case your prebuilt company is using, they'll ship you the extra panel that came with it with your system. If there's a PC you want that only has glass on the side panel, look into the details of the case itself. It might be one of those.

With all of that said, if I were you, I'd look into prebuilt companies that, in general, have a record of providing quality systems, and then look into turning off the RGB.

For instance:

• HP/Omen
• Starforge
• IBuyPower (although I think it's been a little hit-or-miss over the years? Not sure)
• Maingear (very expensive though)

You may want to look at Linus Tech Tips and Gamers' Nexus. They both review prebuilts.

LTT does it in the form of their secret shopper series, where they show the experience of a complete newbie buying and getting support through the phone.

Gamers' Nexus purchases individual systems and review them in depth.

Both channels make great content and should help you come to a decision.