Going for that nice mid 1990's vibe?

In fairness it's got a wealth of features...it's just ugly as sin.

I've actually been running webmin on my raspberry pi 4 practically every since I got it, the night version of the authentic theme(one of the default) actually looks and feels relatively modern and is updated as well every now and then. My only nitpicks are the obsolete set of tools/panels available (it was the core reason why it took me so much to move from apache to nginx, because there's no nginx panel on the default set)

How's the footprint? Is it usable for investigating problems while the system is strained on resources?

It is not null, I'll tell ya. It's a bit of a tradeoff between cpu cycles and ability to do actual tasks. You can't really investigate much a server with it unless you're investigating a machine in a cluster from a main broker because the panel can and will get sluggish to navigate(there's no good alternative to physical access or ssh in extreme cases), but hey, at least it won't drop you easily, because the frontend responds well to high latency.

It has had a bit of a visual refresh, so it's not as bad as it used to be, although the underlying structure is still very much the same.

It has a couple of decent themes, nowadays.