Meet Landing Page using Flexbox, Grid and Mobile first approach

This challenged turned out to be harder than I'd anticipated! This is my first 'big' project that involves a landing page rather than a component. One of challenges I faced was deciding how to organise my CSS code base. At the moment, it just flows on alongside the HTML structure. However, I'm keen to learn about any techniques of structuring CSS and making it more readable/logical. I'm about to start learning Sass - so I anticipate that would be a good method to ensure the codebase is well thought through and maintainable. I was wondering what others do (especially in the industry). I approached this challenge with a mobile first style, which I think is the golden rule these days (but again would be nice to hear someone else's experiences). One question I had was whether or not it's acceptable to use Grid for mobile layout. I really like it and I feel like it makes life of a developer so much easier! I wonder what others think about this! Any other feedback is very welcome! Cheers!