fix: use full s3 URLs for themes images

[ethanpalm]

Documentation changes

Because the themes cards is an exported component, we have to use full S3 URLs for images.

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful

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Note

Low Risk
Single documentation change affecting only image source URLs; risk is limited to broken previews if the S3 path is incorrect.

Overview
Updates customize/themes.mdx so ThemeCard loads preview images from a fully-qualified S3 URL (https://mintlify.s3...) rather than the relative /images/themes/... path, ensuring images resolve when the component is exported/embedded.

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