Knockout uses the Font Awesome 5 Pro icon system, implemented as a font. This means that any unicode glyph from the font can be pasted as text, provided it's given a class corresponding to its set. Light, Solid, & Brands sets are supported. These classes can be combined with other classes for additional styling.
Icons can also be added as pseudo-elements in custom code using unicode or the unicode glyph.
The icon cheat sheet can be found here.