Is Dark Mode supported by AirBuddy?

Yes, and you can customize this behavior in the app’s settings, under the “General” tab. The selection you make in that tab will only affect the status windows that pop up when opening your AirPods case next to your Mac.

Dark Mode limitations in macOS Mojave

The AirBuddy status window doesn't support Dark Mode in macOS Mojave because that version of macOS doesn't support HEVC video with alpha.