theme switch button changed according to the requirements

[Manasi200609]

This Pull Request improves the usability of the theme switcher button by updating its behavior to indicate the theme that will be applied after interaction, rather than the currently active theme.

Previously, the theme switcher displayed the icon or label corresponding to the active theme (e.g., a sun icon when already in Light Mode), which could be confusing for users. This change updates the logic so the button reflects the next theme state, making the interaction clearer and more intuitive.

Specifically:

• When the site is in Light Mode, the button displays a Dark Mode indicator (moon icon 🌙 / "Dark").
• When the site is in Dark Mode, the button displays a Light Mode indicator (sun icon ☀️ / "Light").

Resolves #<431>

• ✨ Content Enhancement — Updates or improves existing content

• 🐛 Bug Fix — Fixes a documented issue or incorrect behavior

• ✍️ New Content — Adds new chapters, examples, or significant sections

• 🛠️ Other — Please describe in the Description of Changes section

• 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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[Manasi200609]

Hello !
I’ve reviewed the current behavior of the theme switcher and proposed an enhancement to improve clarity and usability.
The update modifies the button logic so it indicates the theme that will be applied after clicking, rather than the currently active theme. This follows common UI/UX best practices and reduces user confusion.
I’ve opened a Pull Request with these changes.
Kindly take a look when you have a chance — feedback is very welcome.