Only toggle menu display state on hover

We don't need to toggle padding on hover anymore because we're not
worrying about CSS transitions for no-js mobile.
Only align the nav link in the center

No need to center-align all of the menu content as well.
Rework the dark theme

It's not much, but at least we have one theme that's now mostly
compatible with the new classes.
Merge branch 'master' of https://github.com/mlms13/Navtacular