Piccolo-theme

If ``html_short_title`` is in ``conf.py`` then this is used in the nav bar instead of the full project title.

Fixed dark mode styles - some elements weren't visible. Thanks to alorence for reporting this issue.

Added table styles.

Improved the appearance of autodoc output for C++ files (when using [breathe](https://breathe.readthedocs.io/en/latest/)). Courtesy thijsmie.

Added dark mode 🧛‍♂️

<img width="1350" alt="Screenshot 2022-02-26 at 15 50 14" src="https://user-images.githubusercontent.com/350976/155850245-4b0b9467-4208-4728-97e8-abba0eb55c06.png">

And also a 'darkest' mode, with pure black backgrounds, for people with OLED displays.

<img width="1350" alt="Screenshot 2022-02-26 at 16 09 43" src="https://user-images.githubusercontent.com/350976/155850296-f7b746cf-2f7e-4232-9c55-8c192fa89f2f.png">

Improved search styles.