Explain how to apply custom styling to components in the documentation

[freddyaboulton]

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Is your feature request related to a problem? Please describe.
As a gradio user, I would like some guidance on how to apply custom styling to my Interface or Blocks app.

Currently, we mention custom styling for interfaces in the "advanced interface features" guide but we don't really explain how to use it or give an example:

For blocks, we have a blocks_style demo but as far as I can tell, it's not used on the website.

Describe the solution you'd like
We should document the style method for components on the website and link to examples. We should also write a guide on custom styling for blocks and interfaces to educate users on this feature.

Additional context
Add any other context or screenshots about the feature request here.