Some App icons not displayed. Need failover if your icon set doesn't have entry for the app being displayed

I'm using the Tango icon theme. It doesn't have entries for Thunderbird or Thunar. As a result, I get a generic icon that looks bad and doesn't help me find out what's running. But this sort of thing is happening with those programs in a number of icon sets, and also any app that isn't held in the icon set selected. There needs to be a mechanism to failover to another icon set or to manually set an icon. Some icons look better in the docked tray than others.

I propose using the mechanism that the launcher uses. You can right click a launcher to edit, and have multiple icons and command line settings. Maybe a right click on a docked item could bring up the ability to select icons from another icon set, or to set custom icons? You would need to remember the settings and preserve it in xfce4-panel-profiles.