Restore light/dark custom setting

[aliabid94]

Restores the ability to set the light / dark theme via:

• embedding attribute
• url param "http://localhost:7860/?__theme=dark"

[gradio-pr-bot]

🪼 branch checks and previews

•	Name	Status	URL
	Spaces	ready!	Spaces preview
	Website	ready!	Website preview
🦄	Changes	detecting...

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Install Gradio from this PR

pip install https://gradio-pypi-previews.s3.amazonaws.com/fcb0f6ba174f373ef67acf478c4057582fa93f5b/gradio-5.1.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@fcb0f6ba174f373ef67acf478c4057582fa93f5b#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-npm-previews.s3.amazonaws.com/fcb0f6ba174f373ef67acf478c4057582fa93f5b/gradio-client-1.6.0.tgz

Use Lite from this PR

<script type="module" src="https://gradio-lite-previews.s3.amazonaws.com/fcb0f6ba174f373ef67acf478c4057582fa93f5b/dist/lite.js""></script>

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	patch

• Maintainers can select this checkbox to manually select packages to update.

With the following changelog entry.

Restore light/dark custom setting

Maintainers or the PR author can modify the PR title to modify this entry.

Something isn't right?

• Maintainers can change the version label to modify the version bump.
• If the bot has failed to detect any changes, or if this pull request needs to update multiple packages to different versions or requires a more comprehensive changelog entry, maintainers can update the changelog file directly.

[freddyaboulton]

Tested with both SSR and CSR modes!

[abidlabs]

And I tested on Spaces as well. LGTM, will merge in