The background is intended to reference the portal from which they came in the comic.
https://i.namu.wiki/i/20l1NNLlgvW9G0IekJd7_TvBy3ncSAeOqrU29fLjvMTvaNVvzebSozwVrG761yGRx9A9grz6Ugl92W1HpnwQHQ.webp

You can also find this on Deviantart:
https://www.deviantart.com/mariomaniacyt/art/A-Sense-of-Deja-View-1218756643