missing default icons short ascending / short descending etc with enable-font-icons

[Stel]

Hello,

I am styling a custom classic theme by extending the neptune theme.
I have font-icons enabled.
My issue is that the grid's header menu icons (short ascending, short descending, columns) are missing.
I noticed that if I extend the triton theme, the icons are fetched so I wanted to ask if there is something that I can do to my custom theme in order to display these icons.

Thanks

[Gary Schlosberg]

This page discusses this a bit.
https://docs.sencha.com/extjs/6.0/core_concepts/font_ext.html

Here's a thread discussing one option.
https://www.sencha.com/forum/showthread.php?307445

Or perhaps you could extend the Triton theme instead?