Give a better baseline of CSS

[Alex Dickson]

There is probably a bit of re-inventing with widgets.

We should provide a documented base CSS (think ADG for widgets). We could allow components as well.

[Iván Loire]

In 1.0 you are able to use stylus in widgets, which can access the default stylus color palette defined in Atlaboard core. Next step will be to implement proper theming: https://bitbucket.org/atlassian/atlasboard/issues/27/add-support-for-theming