Thumbnail:

Oh wow. Certainly different from other thumbnail surprise/double images I'm used to seeing.

Rather than having a simple white background with monochrome transparency, this picture is full-colour with transparency on both the background and foreground, giving it two distinct, yet perfectly viewable versions in their own right, for both light and dark mode backgrounds. ☝️🤓

Also, on desktop Twitter in dark mode, hovering over the image produces the effect of the sky glowing.

For anyone really curious, here's what the image would look like with the alpha set to 100%:

And this is the alpha channel itself: asset #24814148

The RGB colour of the transparent part of the sky isn't just the same as the opaque portion with the moon. If it were set to the same colour, then when viewed on a white background, it would appear brighter than it should, as it's being blended with the fully-white background. It needs to compensate for this blending to perfectly produce the full-colour RGB of the sky. The same applies for the transparent bits on Keine.