Let the opportunity to the users to override the CSS of a widget

[loicgreffier]

Is your feature request related to a problem? Please describe.
Let the opportunity to the users to override the CSS of a widget

Describe the solution you'd like
Add a button for widgets which could allow the users to override the CSS of the widget like it is possible to do for the category settings

Describe alternatives you've considered
N/A

Additional context
N/A

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