Does anyone know how to change the preset highlight colour?
If you hold your mouse over the highlight button in the format bar, a little tooltip comes up which tells you to click-and-hold or ctrl-click to change the highlight colour. That gives you a choice of around 64.
Alternately "Format > Highlight, gives you option to choose from a range of colours or show the system Colour pane to create your own completely.
The colour you choose will stick for future highlighting until you change it again.