The main file in any child theme is a style.css file.
Make sure that you have created this file correctly and that the child theme is activated.
Then you can add to this file any CSS rules you like.
All the faucet UIs have classes in the markup. These include the class “wallets-faucet” plus some other classes to differentiate between the different UIs.
For instance, to change the color of the background in the “Faucet earnings” table, you would add something like:
.wallets-faucet.referrals-list table { background-color: lightblue; }
Hope this helps.