Custom CSS is available as an add-on to the pro plan.
Add any number of CSS files to your repository and the defined class names will be applied and available across all of your MDX files.

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Adding style.css

For example, you can add the following style.css file to customize the styling of the navbar and footer. 
#navbar {
  background: "#fffff2";
  padding: 1rem;
}

footer {
  margin-top: 2rem;
}

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Using Identifiers

Mintlify has a set of common identifiers to help you tag important elements of the UI. Some, but not all are listed in the following: #topbar-cta-button #navbar #sidebar #content-area #table-of-contents
Use inspect element to find references to elements you’re looking to customize.
References and the styling of common elements are subject to change as the platform evolves. Please use custom styling with caution.