This just costs me 3 hours of troubleshooting: is it still the case that only one customisation can be enabled at a time? If so, can a simple notice be added on the existing customisation page to point out this bug, because I’m surely not the first or last who’ll fall victim to it?
And as @Sam points out, the UI does mislead you into thinking that multiple customisations work. So I was modulating my CSS, which, as it turns out, Discourse punishes you for [the good practice of].
As a new Discourse user I’ve been very impressed by the platform. But, I’ve spent ten times longer troubleshooting on meta.discourse.org than I’ve spent tweaking my own instance of it. And when you’re setting up a new Discourse and making many changes, it’s not always obvious what has caused a problem. So you search on Meta, find a topic that might be the cause, reply to it (which bumps it up to top of Meta, making others more likely to find it), only to later discover that that topic is totally unrelated to your problem and had the known problem been made known to everyone in the UI then we’d all be having a much better experience.
Why not tell all users what is already known — that only one customisation can be enabled at a time — instead of forcing them to fall into the trap and then find their own way out?