Create a stylesheet for the contributor's role

The top of the “Contribute” page is structured by logical blocks, divided by skills first, and then by role. Some CSS styling should make this logical structure more visible and easy to navigate visually.

The CSS selectors for this section are already defined in the “Contribute section” part of the local.css, only the artwork is missing. Maybe not much is needed…

Parent Task: #6165 (closed)

Original created by @sajolida on 6361 (Redmine)