Projects/Usability/HIG/Buttons

A button initiates an action when the user clicks it.

Toggle buttons are a special case where a button has a state.

Use title style capitalization for the label.
Buttons are not dynamic; their content or labels should not change depending on the context.
Prefer using icons on buttons only for ok, apply, cancel like actions. Passive actions like e.g. those in the "System Settings => Application Appearance => Fonts" don't have icons. Don't overboard with icons.