I installed a new theme called Pury onto my T-Mobile LG G3 running a nightly version of CyanogenMod 13. I am not blaming Pury, but I am just giving specifics. My theory for the problem is that Pury said it was for CM12 but I was running CM13. Though, other themes were CM12 that I had installed but I am not taking that risk anymore!
My phone started giving “System UI has stopped” warning messages. Normally after 5, the theme will be reset to the default theme. That did not happen this time after multiple restarts. Just the same error notification.
To fix this, you need to restart your device into Safe Mode. Other websites suggest rebooting into TWRP and deleting the theme from the disc. I do not believe this is necessary.
On a LG G3, to reboot into Safe Mode, restart your phone normally. Once you see the splash screen, hold down the Volume Down button until the OS loads. It should say “Safe Mode” on the bottom left. Now you can uninstall the app that caused your error message.
Note – When I restarted into Safe Mode, aCalDav and DavDroid had their Accounts removed on my phone. I had to create the accounts again.