That's a "feature" of SVGs. The image is an SVG and since they are transparent it goes dark when switching.

It’s not just that the image is transparent though, because the color of the text changes too. I thought perhaps it’s using the site CSS to style SVG elements (which appears to be possible, though I haven’t tried it), but it turns out it’s just a CSS filter `invert(83%) hue-rotate(180deg)`. Still, it looks great and I’m impressed by the attention to detail.

Ah, serves me right for not seeing it myself and assuming it was just the background. Thanks for the additional info.

Okay, this is driving me crazy. Where?

Bottom-left of the article linked in the OP, not the linked SVG image itself.

It's a very low-contrast sun in the bottom left corner.