I've looked under 'help' in my daemon and wallet and found nothing that seems to describe how to background mine. Is that only a function in the GUI?
Background mining is done by adding "true" at the end of the start_mining command in monero-wallet-cli, after the number of threads. This means the number of threads is not optionalk in that case:
start_mining 4 true
This will start background mining on 4 threads.
If you want to mine, you can do so from the wallet: