To set the icon in field triggers you will have to use the ‘iconCls’ property. Unfortunately there is a bug in the classic-theme (it works in triton-theme). In the override you can see new property ‘glyph’ and new method ‘setGlyph(glyph). The last one will let you change the glyph pragmatically.

The size of the glyph fonts is calculated automatically after rendering the trigger element in the  ‘afterFieldRender’ method.

The override:

Usage example:

 

As you can see you can change the trigger icon programmatically.

Fiddle: