[NO-ISSUE] Add semantic label for light/dark theme toggle button

[TixieSalander]

Proposed changes

• Add missing (empty) alt attribute on the theme toggle's images
• Add a semantic/accessible text label (with an aria-label attribute)

I manually added the aria-label attribute but since more ARIA attribute should probably added in the future, maybe a package making all the ARIA attributes globally available into lucid templates could be an idea.

Contributor checklist

• [x] My PR is related to \
• [x] I have read and understood the CONTRIBUTING guide
• [ ] I have inserted my change and a link to this PR in the CHANGELOG
• [x] I have updated documentation in ./docs/docs if a public feature has a behaviour change Not needed

[tchoutri]

Yep, lucid-aria will just have to be made compatible with lucid2 :+1: