Apply theme to nav bar

[andrewbanchich]

It would be nice to have the nav bar apply the same theme as the documentation itself.

Current style with ayu theme:

After proposed change:

I'd love to contribute this if that sounds good.

[jyn514]

Duplicate of #918

[Cldfire]

@andrewbanchich thank you for opening this though! I for one am very eager to see the navbar colors align with the theme, it really is an eyesore at the moment.