Fix issues no. 431: modify theme switcher logic to show next theme icon

[swati99gup]

Summary

This PR fixes issue #431 by updating the theme switcher UI so the button displays the mode that will be applied when clicked (i.e. it now shows the next theme icon). Previously the button showed the current theme, which is confusing.

What I changed

• Updated theme toggle logic so the button shows the target mode (sun icon for switching to light, moon icon for switching to dark).
• Fixed footer logo filename and visibility classes so correct logo appears in each theme.
• Added a small script to keep the toggle icon and logos in sync when html[data-theme] changes.

Why this fixes the issue

Users expect a toggle to indicate the action that will happen when pressed. Showing the opposite icon reduces confusion and follows common UI patterns.

Testing

1. Open site in light mode — the theme toggle should display the dark icon (moon).
2. Click the toggle — site switches to dark and the toggle now displays the light icon (sun).
3. Open site in dark mode — the toggle should display the light icon (sun).
4. Verify footer logos show the proper variant for each theme.

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