fix(theme): light & dark mode checkbox doesn't announce state switches

[andrewasche]

Pre-flight checklist

• [X ] I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Test Plan

yarn start
open webpage and use nvda to test that the aria-pressed is saying if the button is pressed or if the button is not pressed

Test links

https://deploy-preview-10585--docusaurus-2.netlify.app/

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🟠 80	🟢 98	🟢 96	🟢 100	Report
/docs/installation	🟠 58	🟢 97	🟢 100	🟢 100	Report
/docs/category/getting-started	🟠 73	🟢 100	🟢 100	🟠 86	Report
/blog	🟠 63	🟢 96	🟢 100	🟠 86	Report
/blog/preparing-your-site-for-docusaurus-v3	🔴 48	🟢 92	🟢 100	🟢 100	Report
/blog/tags/release	🟠 63	🟢 96	🟢 100	🟠 86	Report
/blog/tags	🟠 74	🟢 100	🟢 100	🟠 86	Report

[andrewasche]

I have completed the Contributor License Agreement after the workflow run into that issue. I couldn't figure out dogfooding either.

[slorber]

LGTM thanks

Apparently it's not really "needed" considering the aria-label is dynamic, but it also doesn't harm so let's merge it