There’s a variety of system themes built-in:
You can start with one of those or customize your own theme:
The example shown at the very top of the thread is a highly customized theme with lots of Custom CSS.
I would generally recommend using the standard theming features where you can and only fallback to reviewing the Custom CSS Community Wiki if you have a very particular need. With the introduction of features like Super Tiles and many other improvements / enhancements, the most common use-cases are likely covered with built-in features.
As for a feature request around community theme sharing, there hasn’t been a lot of interest in it, so it’s not something we’ve prioritized. We use the official Feature Requests category in the community to help prioritize what we work on. The closest thing I can find there is Share Dashboard Theme and Skeleton which hasn’t picked up many votes.