Update header design

[remibetin]

This PR:

• updates the header design to match the updated WAI website theme
• removes in css/base.css the blocks that could come in conflict with the theme stylesheets.

To discuss with @kfranqueiro:

• There is no cache busting at the moment. This seems needed whenever we update the theme.

To do before merging:

• Point to the style.css stylesheet in production (the current path is used for the preview only): https://www.w3.org/WAI/assets/css/style.css

⚠️ Important: This update needs to be synced with the release of the theme update.

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