Customisation to a template CSS Document

We're integrating a payment page into out website with a payment provider to enable us to take card payments online. They have a standard CSS Document which we need to make some changes to (Colour, Logo). We have a full specification for the CSS and what/how to change.

We're looking for someone that will be able to take a CSS file for Mobile, Tablet and Desktop and make a few modifications, making sure there is no external links for any files and then returning to us to submit. You'll be provided with the image, colours and anything else you need along with the guide on what needs to be changed.