Improve button consistency across light/dark mode

[hannahblair]

Description

The button border settings were feeling inconsistent in light and dark mode, with no border in light but a border in dark mode, and a hover border being lighter than the default fill in dark mode and darker in light mode. tl;dr this makes button borders better

Closes: #(issue)

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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/ddb5a9e698ebe40a47a96513876aa1e37593a7c4/gradio-4.42.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@ddb5a9e698ebe40a47a96513876aa1e37593a7c4#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-npm-previews.s3.amazonaws.com/ddb5a9e698ebe40a47a96513876aa1e37593a7c4/gradio-client-1.5.1.tgz

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	minor

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With the following changelog entry.

Improve button consistency across light/dark mode

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[hannahblair]

merging as the failing tests are coming from 5.0-dev