Like said in the documentation regarding Styles properties, "text buttons use two styles by default: button for the button itself, and button_text for the text inside the button". More basically, there's the style assigned to the button and, derived from it, the style assigned to the text.
So, for something like :
Code:
screen blabla:
textbutton "abc" style "myStyle"
You'll have "myStyle" which apply to the button, and "myStyle_text" which apply to the text inside this button.
In the end, to change the font of the text button:
Code:
style myStyle is button
style myStyle_text is button_text:
font "MyFont"