If you can't select all the way down to 0 or 1 sat/b (as has gotten nextblock confirms when I looked at recent blocks within the last day), it's not 100% custom.
And they support this. They always had the option to allow you to manually customize the exactly amount you want to pay on fee with a textbox, and they also have a option to enable dynamic fees, where they will give you a few options based on calculations on their side - just like I said above.Tools -> Preferences -> check "Edit fees manually";
Thank you for clarifying this, pardon me earlier.
But I much prefer use mycelium for faster confirmations tho, and electrum for multiple sending.