Fix theme switcher to display next theme state

[afsheen-nadaf]

Description of Changes

Fixes #431

This PR updates the theme switcher button so that it displays the
theme that will be applied after clicking, instead of the current
active theme. This improves clarity and aligns with common UI/UX
best practices.

Related Issue

Fixes #431

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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