Update theme toggle button position in CSS from top to bottom

[healthkowshik]

Motivation and Context

When trying out the example apps, I noticed that the light/dark theme selector was overlapping with the x icon of the app on the top right corner.

How Has This Been Tested?

git clone https://github.com/modelcontextprotocol/ext-apps.git
cd ext-apps
npm install
npm start

Then open http://localhost:8080/

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed