I would like to add an administrative page that allows an admin to set the default site theme instead of setting it in the web.config file. My goal is to give the site admin the option of selecting/changing the default theme at their discretion. Any guidance will be most appreciated!

No reason you can't do both. Have a page that sets the theme using the .NET Configuration API. That way, you can leave it stored in web.config, and change it from your page.

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