Nav Block: Inserter color should always retain contrast to the background color

[apeatling]

Describe the bug
When using a dark background color on the navigation block, the inserter becomes difficult to see:

I think it should either always adjust itself to be a color that is high contrast to the selected background color, or match whatever the text color is.

To reproduce
Steps to reproduce the behavior:

1. Add a navigation block with some items
2. Set the background color to a darker color

Expected behavior
The inserter is always a color that is high contrast to the background color.

[apeatling]

Should be fixed by #18327

[karmatosed]

This has now changed with recent iterations to a uniform '+'. For now, closing but we can always reopen.