Added the ability to define custom icons for the system tray. (#408)

* Added the ability to define custom icons for the system tray.

* Add placeholder icon if icon is not defined.

* Update themes

themes/cyberpunk_vivid.json

@@ -340,5 +340,6 @@
   "theme.bar.buttons.modules.submap.icon_background": "#121212",
   "theme.bar.menus.menu.network.switch.puck": "#333333",
   "theme.bar.menus.menu.network.switch.disabled": "#2A2A2A",
-  "theme.bar.menus.menu.network.switch.enabled": "#FF69B4"
+  "theme.bar.menus.menu.network.switch.enabled": "#FF69B4",
+  "theme.bar.buttons.systray.customIcon": "#d1d1d1"
 }