HighContrast: Restore expander size

Expanders used to be 16px high. With the move from the gtk2 rendering
to gtk3 rendering they shrunk to 12px, making them hard to see, because
it's now the icon which is 16px high and the icon contains transparent

This makes the HighContrast theme use 24px icons instead, to restore
16px expanders. This may expander some containers a bit.

Closes #1046
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