Improve Styling doc

[FlorianJacta]

Styling documentation is confusing. Users have a hard time finding it and make the most out of it.

Possible solution

We should put the Styling doc just below User Interface:

• A sub-section should present how to style an application,
• A sub-section should present how to style controls (putting text in red, adding margin, padding, and more).

This should refer to a Stylekit page that shows every class of the Stylekit.