Hi! The feather icons currently included in the theme are all included as SVG. So to add other feather icons you have two options: get them as SVGs or add the icon font files to the theme:
Add the following between the <head></head> tag:
<script src=“https://unpkg.com/feather-icons”></script>
Then you can add the icons as (for example):
<i data-feather=“circle”></i>
You can read more here: https://github.com/feathericons/feather#feather
And find the icons here: https://feathericons.com/
Hope this helps!