Navigation Bar Custom Icons

[prettyinpossiblee]

I am trying to change the icons used by the Navigation Bar elements:

But I couldn't find any way of changing the icons used by the elements. I know that I can just set custom icons, but I also want to be able to set the custom colors with this GUI. So having just the option of changing them easily like this is much better. Is there a way to change them? I tried looking for the SVG paths, but I couldn't find anything.