If you've ever wondered about the unusual arXiv.org website favicon: The original logo was a skull with bones as reference to the piratey nature of distributing preprints without the publisher's consent. The smiley face was added later in order to make it less offensive. Due to the conversion to a non-transparent format, the backgrond was filled with the green color.

https://www.quora.com/Whats-the-story-behind-the-arXiv-org-f...