Refactor Carbon CSS reusable overwrites

[eddybrando]

We have identified some Carbon CSS overwrites, like the one applied to the accordions in ContentAccordion and StartLocally.

This overwrite (and potentially others) should be reviewed to check the need for more than 1 customisation of the component, and the definitions should be moved to the assets/css/components directory to organise them better and avoid repetition, following the new structure introduced with #965.

[delapuente]

Returning this to a regular level of priority.

[eddybrando]

I believe this issue doesn't apply anymore as we've refactored our codebase since then.