Managing styles

[SachaG]

Right now we have four potential places where colors and other styles can be defined:

• The stylesheet
• The Nivo theme (nivoTheme.js)
• The constants file (constants.js)
• The component file itself

Even without switching to a CSS-in-JS solution, I think we could get from 4 to 2 by deciding on the constants file (for example) as a single source of truth for all colors at least.

To help with that I created a new getColor(id) function (getColor('bars'), getColor('would_use_again'), etc.).