Feature/mixins for decorations

# Decorations

Previously, decorations were only on a case-by-case basis. Now, through practice and varied use, they're now included by default.

- `.bordered` will add a border of default 1px black. 
- `.shadowed` adds a box-shadow with some default styling
- etc.

I've also mixed these styles into (semi-)extendable mixins. The result is less code to maintain and easier ways to create additional styles based on these (e.g. more color options). However, good documentation is a must.