The dialogues provide a richer way of selecting values for some CSS properties, where simple controls cannot display the full range of potential values. They are accessed by pressing the buttons. They are also available as standalone pickers.
The dialogues "talk" to the page that opened them, setting the values that you choose, and retrieving previously stored values. Choosing a different page of controls will therefore stop the dialogue from functioning correctly; if this occours, close and re-open it.