CSS/Layout fixes so base webmaker-nav can be used in Popcorn Maker

The remaining problem in Popcorn Maker trying to use this repo rather than our own version is the fact we run it on two pages. That other page has CSS conflicts with this.

I could write something to fix it on our end for that page specifically but doing so here doesn't break anything for Thimble and potentially prevents future issues when this is integrated in other places.