feat(root): adds dark mode switch

[ad9242]

Summary of the changes

Adds dark mode switch to site. This is an initial PR which can be built upon to fully implement the solution.

not in scope for this PR:

• storing the setting
• detecting os\browser settings for default
• full dark mode of site

Related issue

1216

Checklist

• [x] I have manually accessibility tested any changes, if relevant.
• [x] I have raised this pull request against the develop branch

[github-actions[bot]]

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