Add a text label to the theme dark/light mode-changing button

[samcunliffe]

See parent issue for explanation in James' email.

Definition of done

The mode-changing button has an alt-text or meaningful label: It doesn't say "moon" (or "sun") when read by a screen reader.

Docs

Solution could be an aria-label.

[samcunliffe]

Huh. Should have been auto-closed. Probably I didn't use the correct word.
Solved via #543.

[paddyroddy]

Huh. Should have been auto-closed. Probably I didn't use the correct word. Solved via #543.

You had Fixes

• #num

I think you have to do Fixes #num