Just wanted to share my issue and solution with the community for the next person experiencing this problem.
After updating to 2026.2, my saved theme setting for my user was constantly changing to a theme (Graphite e-ink in my case) no matter what I changed it to in my user profile setting. My preferred theme would stick for a little bit but then I’d log in later on and sure enough, back to E-Ink. I cleared cache etc. but this problem persisted across multiple browsers and the iOS app.
I have the Puppet add-on running for an e-ink display. The display wakes up every hour and gets a screenshot using a url like:
[https://homeassistant.local:10000/home?viewport=1000x1000&theme=Graphite+E-ink+Light&lang=en]
which contains the theme in the request. The token I had set up for Puppet to use is tied to my login (a no-no, I get it) and I could replicate the default theme change every time I made a Puppet request with a theme specified.
I’ve created a new Puppet user and token and this should solve the issue. I am not sure what changed in 2026.2 that causes this new behavior but there ya go.