add custom_css param to theme class

[hannahblair]

Description

Allows embedding custom css into generated themes via a custom_css param on the ThemeClass object. The custom_css is appended at the of the theme CSS output at /theme.css, so will override any theme variable.

• to_dict() now also includes gradio_version so themes loaded from HF can warn if there's a version mismatch (comparing major.minor)
• also adds a CSS variables reference guide generated by scripts/generate_css_vars_docs.py

Closes: #13038
Closes: #13037
Closes: #13039

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[gradio-pr-bot]

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Install Gradio from this PR

pip install https://gradio-pypi-previews.s3.amazonaws.com/099f06a95df67552c7b247c278e967c606b052af/gradio-6.9.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@099f06a95df67552c7b247c278e967c606b052af#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-npm-previews.s3.amazonaws.com/099f06a95df67552c7b247c278e967c606b052af/gradio-client-2.1.0.tgz

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	minor

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• add custom_css param to theme class

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‼️ Changeset not approved. Ensure the version bump is appropriate for all packages before approving.

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[freddyaboulton]

LGTM @hannahblair ! Nice