.fb-btn-plus is the main class, selected this class you can set the padding (from the CSS variables) and border radius of all the buttons.

.fb-btn-plus– are the modifiers and only affect that type of button, to modify the color on hover you must go to the CSS variables.

.fb-btn-plus–small and .fb-btn-plus–big are the size modifiers, they only modify the font size, as the buttons use em you don’t need to change anything else to change the size of the button.