Is it possible to change the appearance of an icon in the system tray and if yes, how?
The icons can be changed in the icon theme you're using, in usr/share/icons/<icon theme>. Tray icons hide in the status folder of that specific theme. Look for the right name of the application, and change the icon in the 16, 22 and 24 pixel folder. Beware that some applications inherit icons from other icon themes. For instance, if you're using the oxygen icon theme, it inherits gnome and hicolor icons if the right name for that application cannot be found in the /usr/share/icons/oxygen/ folder. Which icon themes are inherited is written in the index.theme file of the icon theme in use (usr/share/icons/index.theme > Inherits=gnome,hicolor) which can be changed as well. After changing the icons, run gtk-update-icon-cache /usr/share/icons/<icon theme folder>/ for them to be used.