Implement Styled Components Theming support

[Kingdutch]

When we figure out the color palette (#3) then it may be a good idea to set this up using the Styled Components Theming system. This allows future RustFest events to copy the Rust Netherlands website and easily change colors.

Figuring out the structure for this is never easy because you want color changes to be easy while retaining precise control over buttons.

And example can be seen here:

• Declaration: https://github.com/goalgorilla/react_social/blob/master/app/components/themes/theme.js
• Usage: https://github.com/goalgorilla/react_social/blob/master/app/components/atoms/BaseButton/index.js