Hello @simonb, @Robert_McIntosh,
Yes, the color of those icons can be configured. First of all, though, you should know that the default color for these icons is derived from the header_primary color variable. On the Sphere community, I see that the theme overrides the color generated from that variable, but only for the regular header buttons (which makes sense, the iPad nav is, well, only visible on iPads).
A quick fix would be to add the same CSS you’re using for your header icons to the DiscourseHub nav icons. You would need to target the .footer-nav .d-icon element.
A better fix is to remove the override in your theme, and modify the header primary color variable. That will have repercussions on other header text (titles etc), but it should work.
Let me know how it goes.