Improve accessibility and clarity of theme switcher button

[afsheen-nadaf]

Description of Changes

This PR improves the accessibility and usability of the theme switcher button by providing a dynamic accessible label and tooltip that indicate the theme that will be applied when the button is clicked.

• Adds a dynamic aria-label for screen reader users
• Updates the title attribute to reflect the next theme state

This aligns the accessible behaviour of the button with its visual representation and improves overall clarity.

Type of Change

Please check the appropriate box that describes your PR:

• 🐛 Bug Fix — Fixes a documented issue or incorrect behavior
• ✨ Content Enhancement — Updates or improves existing content
• ✍️ New Content — Adds new chapters, examples, or significant sections
• 🛠️ Other — Please describe in the Description of Changes section

Checklist

• I have searched open pull requests to avoid duplicates.
• I have built the book locally using the build instructions and verified my changes work as expected.
• I have written clear, conventional commit messages, as per the Pull Request Guidelines.

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