I've attached a screenshot of the problem.
The new Bookmark icon shows light colored pixels around the icon if you set a dark colored background in your style.
Also, a feature request:
allow custom bookmark icon or at least an option to use the old 4.x orange icon :(
Steps to Reproduce:
Settings > Styler Configurator > Global Styles > Line Number Margin -- Background color to black
set bookmark (Ctrl+F2)