I cannot tell you the last time I used that menu before this thread.
The images are stored in the theme. All the plug-in type images in the Tungsten theme are dark gray. Checking several releases as far back as 2016.05 when Tungsten was introduced show these images with a black background (or possibly no background) on my PC running Win10. Similar to the image in post #9.
If this is a recent change, it would seem it is code shared by all the version of SONAR. IOW an MS library.
The only resolution I can think of ATM is use a different theme.
Might be worth a
problem report.