I just spent quite a bit of time playing and tweaking CSS and testing it with different screen sizes to make sure it was all responsive. You can learn a lot from the inspection menu in your browser (F12) and using the element picker tool (Ctrl + Shift + C on firefox, top left button on inspection panel), and see exactly what classes elements use, and from there you can make modifications to those classes as needed.
Outlets are also powerful to use, but they can be a bit confusing to navigate.