Add functionality to allow replacing constants files

[Oliver Baker]

As a user of the UI library without much interest in CSS or styling (but who wants to change the color scheme), I want to be able to use a simple color scheme object like the one exported from hs-ui, but I don't want to have to replace every use of every color through a new styled prop. It's unnecessary and I hate styling and CSS.

AC:
Implement a method for replacing the files in the constants directory with custom ones.