I'm pretty sure this bug shouldn't be reported to Breeze - General, but I don't know where it belongs. Well, the problem is pretty easy: user cannot see icons in Note widget depending on the background color of the widget itself and the Dark or Light variant of Plasma Workspace theme.
I lack the knowledge about plasmoids to do this, but it seems like the color of each PlasmaComponents.ToolButton in /usr/share/plasma/plasmoids/org.kde.plasma.notes/contents/ui/main.qml should be changed from white to black.
*** This bug has been marked as a duplicate of bug 353819 ***