Drawing a TreeViewItem on a dark background, I noticed that I always got a dark arrow, which is because LookAndFeel_V3::drawTreeviewPlusMinusBox uses a contrasting colour to the tree view item, which is called only from LookAndFeel_V3::drawTreeviewPlusMinusBox with a hardcoded Colours::white, which will then only ever produce translucent black for the arrow. Could this be configurable in some way based on the TreeView item?
Perhaps a combination of a TreeViewItem::backgoundColourId or a functional equivalent and a virtual getPlusMinusBoxColour method (which defaults to contrasting with the background colour, but can be overriden if you want to specify it manually?