Fix accessibility of theme toggle button

[C-Loftus]

What does this PR change? What problem does it solve?

• The button to change the theme has title Alt + T
• this gives it a good tooltip on hover but it a screen reader user would just hear Alt + T and not know what the button does.
  • this is important to add since it is on the front page of every papermod site
• Adding aria-label="Toggle theme" makes the screen reader speak out Toggle theme which is the semantic purpose of the button without overriding the title or tooltip

This isn't perfect and the button could have a white rectangle around it as well for focus but wanted to just add this since it doesn't require me going into the CSS as a first PR

Was the change discussed in an issue or in the Discussions before?

Not that I am aware.

PR Checklist

• [x ] I have enabled maintainer edits for this PR.
• [ x] I have verified that the code works as described/as intended.
• [x ] This change does not include any CDN resources/links.
• [x ] This change does not include any unrelated scripts such as bash and python scripts.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[C-Loftus]

Not clear why merging is blocked, I just did a git push on my new fork and opened a PR on the github UI