Move theme toggler to a partial

[XhmikosR]

Description

Motivation & Context

Type of changes

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

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-39807--twbs-bootstrap.netlify.app/

Related issues

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TODO:

• Fix toggler in examples

[XhmikosR]

I plan to move all this inline CSS to an external file and inline it after processing it through Sass. That's for later, though.

[julien-deramond]

Just a minor modification in comment. The rest LGTM. It's a good idea to make this theme toggler a partial.