Add more opinionated snippets to respective PHP and CSS files for people to copy-paste

[mrwweb]

I have some styles that feel just beyond the "baseline" of the plugin but I want to put in here. So I think it makes sense to put those into two new files, one for PHP and one for CSS.

It would be good to try to write out some sort of limiting principle before that to help me determine what is and isn't in-scope.

For CSS, it should pretty much always be focused on stripping away and paring down rather than changing the design in opinionated ways.

PHP is a little more ambiguous, but the focus should also be on stripping back and only changing defaults where the defaults really seem strange.