Setting theme options should not force base to light

[w-tv]

Checklist

• I have searched the existing issues for similar feature requests.
• I added a descriptive title and summary to this issue.

Summary

Consider https://docs.streamlit.io/develop/concepts/configuration/theming

I'd like to use primaryColor to change the accent color of my app. However, it works equally well in light and dark mode, and I would like users to continue to get their system defaults with regard to light mode / dark mode.

Currently, ' If base itself is omitted, it defaults to "light" '. While it's nice that this is documented correctly, the behavior is undesirable. Instead, both themes should be modified, unless a base is specified.

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