I'm thinking the fact that double clicking in the CSS Designer does NOT open the CSS Rule Definition box is a bug. There is a workaround, even though it's a bit tedious to have to move your mouse across the whole screen instead of just double clicking. Click on the rule you want to edit in CSS Designer, then on the left side of the property inspector (bottom of the screen), click 'Edit Rule'. Voila, this will give you the CSS Rule Definition box.
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