CSS Restyle and Dark Mode for an Existing Website

I need a front-end designer to improve the CSS of an existing website for my Android finance app. The site is already built and live (plain HTML/CSS with a small amount of JavaScript, around 12 pages). The structure and content are in place — what I am looking for is a designer's eye on the styling.

Scope of work:

1. Modern visual restyle — refine the colour palette, typography, spacing, buttons, cards and tables so that every page looks clean, professional and visually consistent.

2. Dark mode — implement a proper light/dark theme with a working toggle, built on CSS variables so it is easy to maintain.

Requirements:
- CSS only. Please do not alter the HTML structure or the JavaScript behaviour. If a small markup change is unavoidable, confirm with me first.
- The result must be fully responsive and look correct on mobile, tablet and desktop.
- Clean, well-commented CSS using variables rather than hard-coded values. No heavy frameworks or build tooling.
- Deliver the updated stylesheet(s) along with a short note explaining what you changed.

I will share the page files with the selected freelancer. The budget for this work is fixed at INR 1,000.

In your proposal, please tell me how long you will need and include one or two examples of websites you have styled. Proposals that are clearly copy-pasted templates will be ignored.