Fix issue #1399: Scroll-to-top button styling for dark theme

[Darshini240906]

📌 Description

Fixes #1399

Fixed the dashboard scroll-to-top button so it renders as a styled circular button instead of a plain white bar. Updated the component to use a dedicated CSS utility class and added theme-safe styling for dark mode.

🛠 Changes Made

• Updated ScrollToTop.jsx to use scroll-to-top-btn
• Added .scroll-to-top-btn styles in index.css
• Improved hover and focus states
• Ensured proper appearance in both light and dark themes

✅ Checklist

• Followed project structure
• No console errors
• Linked the issue
• Properly tested changes (tick if tested)
• Code runs locally (tick if verified)

🚀 Notes for Reviewers

Please verify that the scroll-to-top button:

• Appears as a circular button
• Remains fixed in the bottom-right corner
• Displays the upward arrow icon correctly
• Works as expected in dark mode