Although I’ve long had a habit of frequently changing my themes, even having a key chord set for that, at some point recently I became unable to switch to any light-background theme; it was always some shade of black. This became a real problem since Wikipedia in eww became unreadable for equations. I looked into this and found the answer from Sacha Chua, who pointed me to M-x customize-face default which had somehow had my default background set to some dark dark gray (I’m guessing when I was playing with persp mode).