Add container style option

[hyperfekt]

The look and feel of the site is influenced by what surrounds containers to a large degree. If we let users set this in an option, they can easily choose their preferred style (for example 8px black border (new design) vs 2px box-shadow (old design)). With the adaptation to the new design many more places use the container style directly and will adapt automatically, but this should be expanded to wherever possible.