fix: mount_css missing base path

[chunlampang]

Description

Someone just closed my issue without checking carefully: #11075

[abidlabs]

Hi @chunlampang thanks, I believe @aliabid94 is working on this fix in parallel. @aliabid94 can you address this PR?

[chunlampang]

Hi @chunlampang thanks, I believe @aliabid94 is working on this fix in parallel. @aliabid94 can you address this PR?

I think he (#11165) is only focus on theme.css, but did not handle other css files (such as the example in my issue).

[gradio-pr-bot]

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[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/core	minor
gradio	minor

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With the following changelog entry.

fix: mount_css missing base path

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