Refactor CSS to enable dark theme

It's looking pretty snazzy.

Screenshots: https://meta.discourse.org/t/its-a-dark-theme/15662

![](https://meta.discourse.org/uploads/default/5007/fd4782927707f5e9.png)
![](https://meta.discourse.org/uploads/default/5009/71111aefc5b9b622.png)
![](https://meta.discourse.org/uploads/default/5010/2452a8de21ecaf43.png)