Modern Bootstrap Redesign With Theme Toggle

I have a fully-functional PHP application whose interface still relies on an early Bootstrap release. All the back-end logic works well, so the job is purely about giving the site a fresh, modern and vibrant face without touching the core functionality.

The area that needs the biggest lift is the overall visual appeal. I’d like you to replace the current markup and styles with a contemporary layout—feel free to move to Bootstrap 5, TailwindCSS or a comparable framework if that speeds things up and keeps the code clean. Every existing page, form and message should remain where it is in the flow; only the look and feel should change.

Colour isn’t fixed, but visitors must be able to switch between a light and dark theme with a simple button. Store the preference in a cookie so it persists across sessions.

Beyond that baseline, you’re welcome to suggest small UI/UX enhancements. One idea on my list is letting users attach files to their comments; if you see other quick wins, outline what you’d add and why, and I’ll authorise any extras that make sense.

Deliverables
• New HTML + CSS/SCSS (and any JS) for every existing view
• Light/dark theme toggle with cookie persistence
• Brief read-me explaining any new assets or build steps
• Optional: proposal document for agreed enhancements

The codebase is version-controlled, so you’ll receive a branch to work on and can issue a merge request when you’re done. Let me know your estimated turnaround and, if you plan to introduce any libraries, which ones and why. Around 12-18 windows but only one css file. Sample screenshots are attached.
Simply put, we're changing the CSS file.