The Edit CSS Rules Pane
The Edit CSS Rules pane allows you to edit rules for the currently selected element or element type (this element or type is selected with the Choose Parent and Choose Target Child menus).
Edit the rule values (or set to blank to delete the rule) then click the "Save Rules" button.
Default Rule Values
Every form is preset with default values for margin and padding (both 0) for some elements and element types. Because various browsers apply default margins and padding differently (and set these defaults to different numbers), margin and padding are zeroed out to give you a clean slate to apply or not apply margin and padding as you see fit.