add custom_css param to theme class

[hannahblair]

Description

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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/fe52cc21e2c3213d702066a69f10ceeef053bfa8/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@fe52cc21e2c3213d702066a69f10ceeef053bfa8#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-npm-previews.s3.amazonaws.com/fe52cc21e2c3213d702066a69f10ceeef053bfa8/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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