There's gotta be something else going on, a normal app is unable to go modify a theme globally since it's running in its own sandbox.
Connect for Lemmy App
A community for the mobile app Connect for Lemmy.
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It shouldn't happen, but for me it is. Lock screen clock has turned lime green, Messages app is now using mint and lime colors, and other random bits of green coming up other places. They're the default colors shown for Connect, and weren't part of my global color scheme until I hit the save button this morning. If Connect can affect volume control buttons for other apps outside it's sandbox, then affecting color settings doesn't seem to be out of the question. At least it's a minor issue, just unusual.
I changed the Connect colors just now to something garish, but the phone OS theme hasn't changed, nor the colors in other apps. I would check your OS theme settings.
I reset my phone colors and those in Connect, and can't get it to repeat what happened before. The color settings weren't ones I would have consciously chosen, and didn't pop up until after checking out the Connect options. Dunno. Maybe my unconscious self is now getting screen time and changing stuff around, just to fuck with my conscious self. 🤪