Some accounts, not all, recently defaulted back to the "blue" theme settings. I received a few inquiries from teachers on this so I figured I would share the response with everyone.
According to Google:
"This was NOT a planned feature change to Classroom, but rather a bug. The team is hard at work to fix it and we will report back here as soon as it is resolved." Using the colors to differentiate your classrooms is important and we will get it back, just need to wait for Google to sort it out. Hopefully this will be sooner than later. Have a great day!