fix(theme): unify light/dark theming + header banner + ScrollToTop UX🐞

[ayushpandey101]

Which issue does this PR close?

• Closes 🐞Light/Dark toggle inconsistencies + header banner theming & ScrollToTop UX improvements🐞 #222 .

Rationale for this change

• Fixed inconsistent Light/Dark theming on header banner and ScrollToTop button.
• Persist theme across reloads/navigation and avoid flash of incorrect theme.
• Improve accessibility (contrast, focus visibility, reduced motion) and prevent ScrollToTop overlap on small screens.

What changes are included in this PR?

• Centralized theming via data-theme on html and CSS variables in index.css; removed hardcoded colors.
• Theme persistence (localStorage) with prefers-color-scheme fallback; initialized on load.
• Header.css and ThemeToggle.css updated to consume CSS vars; contrast and spacing adjusted.
• ScrollToTop.jsx themed via vars; added aria-label/focus styles; smooth scroll with reduced-motion support; z-index/position tweaked to avoid footer overlap.
• Affected files: ScrollToTop.jsx, ThemeToggle.css, Header.css, index.css.

Are these changes tested?

• Manually verified theme toggle, reload, and route changes (no flash; state persists).
• Checked small viewports: ScrollToTop doesn’t overlap content/footer.
• Keyboard/screen reader: focus visible and aria-label announced.
• prefers-reduced-motion respected for scrolling.
• Lint passes; no console errors observed.

Are there any user-facing changes?

• Yes: consistent Light/Dark theming across header banner and ScrollToTop; improved contrast and focus outlines.
• Theme preference is saved and defaults to system setting on first visit.
• ScrollToTop placement/visibility refined and smooth-scrolling enhanced.
• No breaking changes; visual/UX improvements only.

Screenshots

Dark Mode:

Light Mode:

[github-actions]

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