When viewing the site with dark mode enabled, the Haskell logo at the top displays with a white background:
From some initial skimming of Discourse docs, it sounds like there may be support for separate logos when either light or dark mode is enabled. If not, we could likely use the same logo but with the white background removed. This is how that looks from local testing:
I’ll attach the test image I used, but fair warning that the background removal on it is imperfect: