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 borders.
This makes the HighContrast theme use 24px icons instead, to restore 16px expanders. This may expander some containers a bit.
Closes #1046 (closed)