* Ensure footnotes classes generated are not filtered
pulldown-cmark add somes classes when footnotes html is generated. This commit ensure they are not filtered by html sanitizer
* Add some footnotes styling
* Theming
- Custom CSS for blogs
- Custom themes for instance
- New dark theme
- UI to choose your instance theme
- Option to disable blog themes if you prefer to only have the instance theme
- UI to choose a blog theme