Ruby on Rails Expert

Hello, I need an expert in Ruby on Rails + React.JS for make a new functionality in an open source project. The feature that I need is have a 'Dark' theme in the web application. Right now it's only light theme. By private message I will sent you the link of the open source project.

The application is in react.js in the frontend and ruby on rails in the backend.

I need know how long it will take.

Thank you.