Qt: ok/cancel button design in toolbar editor

The design of the ok/cancel buttons at the bottom of the toolbar editor is imperfect.

• The text is not centred vertically in the grey box.
• There is no gap between the two buttons to know where one ends and the other begins.
• The focus border only covers an odd portion of the grey box.
• The very faint grey background colour and lack of hover/unfocused border colour makes it difficult for users to tell that these are buttons.