Sorry about that, ** I never read about your problem in this forum, so, it's not a common issue.
The basic troubleshooting steps, in such a situation, are:
Force restart the device, to see if there was a temporary graphic glitch (but you performed that procedure to no avail); next step is to restore your iPhone to factory settings.
This procedure automatically installs a fresh copy of latest iOS version (in case something went wrong during the update process). To do this, make a full backup of your device, then restore it to factory settings and, in the end, set it up as new, according to these instructions:--> Restore your iPhone, iPad, or iPod to factory settings – Apple Support
When done, set the dark mode and test your iPhone for the keyboard look using SMS or Notes, WITHOUT restoring data from backup. If everything is okay, restore data from your previously made backup.
If, after restoring data, the issue comes back, your data could be corrupt.
Regards
Giulio