Template and draft formatting changed overnight

My newsletter went out this morning with formatting anomalies that weren't present when I scheduled it yesterday. I checked and all of my templates and drafts—frequently used and rigorously updated—had changed across the board. Spacing in body and headers went from 1.2 to 1.5, buttons went from Arial to Open Sans, body links took an underline which wasn't present before. What's going on? I've reverted my templates back to the correct style but I really hope this isn't going to become a persistent issue.