Is PMMail still be developed?
The answer is yes, and no. Currently, there is no developer working on it. I, and a few others, tweak it sometimes, when we can, but I am not a programmer, so I can't do much with it.
No promises, but if it is still being maintained, it would be worth while to try to update the graphics.
The icons were tweaked years ago, and that turned into a bit of a marathon, to try to get them to actually work properly. The original icons, in PMMail before 3.x, were not standard OS/2 icons, and a lot of the code needed to be updated to handle normal icons. Most icons were created from the non standard ones, so they didn't change much. At least they are now proper OS/2 icons.
There is a section in the help, about customizing icons. I suggest that you start there. If you have some success, open an incident in the bug tracker, and I will see if I can get it done. The icons seem to be something that I should be able to work with.