Accidentally sent THETA/TFUEL tokens to a Guardian Node address, instead of staking the tokens to the GN address

You can recover those tokens. You just need to get the private key of the Guardian node itself you sent the tokens to, then import it into the Theta web wallet. First you need to find the encrypted keystore file for your GN, which should be in this folder:

on PC: C:\Users\(your username)\AppData\Roaming\Theta Guardian Node\ThetaGN\configs\mainnet\key\encrypted

on Mac: /Users/(your username)/Library/Application Support/Theta Guardian Node/ThetaGN/configs/mainnet/key/encrypted

You can search your hard drive for 'Theta Guardian Node' to help locate this folder

There will be an encrypted keystore file in that folder. Don't open the file, but instead go to the Theta web wallet and click "Keystore". Click 'Choose Keystore File' and pick that encrypted keystore, then enter your node password where it says "enter your wallet password". Then you should have your node address open and see your tokens, and you can send them back to your regular Theta wallet.