Extract box shadow CSS rule to a CSS variable

[rkclark]

box-shadow: 0 0.1rem 0.4rem 0 rgba(0, 0, 0, 0.4) is used 13 times across the project.

It should be included as a CSS variable in the src/css/units.css file and then imported into the files where it is needed.

[mattpaulwilliamson]

@rkclark I will take this issue if you don't mind. Thanks