Basic style updates to improve responsiveness

This is a really cool project!

I noticed the responsiveness was not quite right, so I fixed class names and moved a few small things around to make it look a little better on <768px screens. In this PR:

* Improved the class naming across the site to break at 768px
* Added the `meta` tag to actually make the responsive style kick in
* Fixed a small misalignment issue in mobile navigation

Nothing functional changed, and anything that was moved in the code was done purely for a style improvement.