As more sites and apps support dark mode, I've seen tons of logos and profile avatars saved as transparent PNGs and SVGs begin to break. The fix is simple, though: make sure that your art has a border in a contrasting color. If your art is all black (the most common situation) or otherwise mostly dark, a thin white border will be invisible on a light background, but will set off your art nicely on a dark mode background. Similarly, if your art contains a lot of light color, a dark border will prevent it from washing out on light backgrounds.

Below is my example for both cases. Notice how I add a 10-pixel white border on the first logo to suit the art, and a 1-pixel border and a drop-shadow to the second logo to keep it subtle since the art is more complex:

That's literally all I have to say! I've seen this a lot in the wild and it's apparent it bears to be mentioned. I hope you didn't need the help, but you're welcome if it did ;)