I've recently noticed that the issue with using dark themes was resolved. Now QR code gets displayed properly when you use dark themes on your operating systems as well.
I've also noticed some people complaining that QR codes are unscannable. These comments were made a while ago and I can't vouch for this extensions for time before I made my first comments, but I've tried two apps:
* Qr Code Reader (me.scan.android.client)
* Xiaomi's default scanner app
The codes are readable by both (although Xiaomi's app is worse at scanning QR codes in general), so really. I don't have any (more) complaints.