Change the system icons/graphics?

Just wondering if it’s possible to change the icons/graphics for EM Client? I’m using the Dark Theme but the new mail tray icon is difficult for me to notice so I was hoping to adjust it. I tried saving the theme so I could customize it, but this doesn’t allow me to change the graphics, only the colors. Also, I couldn’t find the New Mail Tray Icon in the theme anywhere so I can’t even change the colors.