More configuration options come with an increased risk of inadvertently pointing one of the gun's barrels at ones own foot without noticing and then pulling the trigger.

To stay with the theming example: it's nice and dandy have control over theme colors. But what happems when you set button background, window background and text color to white by accident? How do you recover from that? Back in the 90s you'd have been stuck with a mostly unusable UI. Somehow that was OK.

Nowadays, things like that are tolerated a lot less. Configuration options in mainstream products are (often) fussed over to the nth degreee to make sure that there isn't a way to get the whole thing into a bad state. The result is usually somewhat more robust. And that's enough to satisfy most users most of the time.