Links preview: add CSS variables to customize its look and feel

[humitos]

We've received a support request from a user wanting to change the background color of the links preview because their text is white, so it's not possible to read it when displayed on the popup.

This work is related to:

• More variable fixes #487
• Add styles based on the documentation tool #473
• Add guidelines on attributes and CSS variables #483

This issue could be a good opportunity to try the pattern we are building here.

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