I think it's a combination between Dark Geoscape mod and the update of OXCE. It's not a save. It's simply clicking on new battle in the main menu. All mods disabled except Dark Geoscape in the new version will result in this. In 7.7 it looks fine. I checked the mod. It's the same version and the file has the same size and timestamp, so the mod should by all accounts and measures be identical. The only variable is the OXCE version.
EDIT: Sorry, yes you're right. I actually haven't turned off the x-com-files properly. If I turn that off, the first 7.7 screenshot looks bad too.
EDIT2: After some more debugging, I found the culprit: Resources/Pals/NEW_XCF_PALLET.pal (part of the X-COM-Files mod). Deleting that from the customPalettes resolves the issue.