How to apply a custom style to a button?

Hi,

I'm guessing this is going to be a very easy solution but I can't figure out what keyword to even look out to find the answer I need. How do I apply a custom style to a button?

I created a theme, added a button, then I can create a new Stylebox for its different states : normal, hover, focus, disabled. I can add a custom state, let's say "locked" but how do I apply it to my button? I can set disabled on true or false by code, and the correct theme is instantly loaded. But how do I tell Godot that he has to load my custom theme for the custom state "locked" existing in my theme.tres file?

Thanks in advance!!