Silex is desktop first. Styles you set on Desktop apply everywhere. Switch to Tablet, Mobile Landscape or Mobile Portrait and each change is wrapped in a max-width media query: 992px, 768px, 480px.
So you only override what breaks, and the rest carries down from Desktop.
The breakpoints page explains this cascade, with a 41-second video. If a step doesn't match your editor, the "Edit this page on GitLab" link is at the bottom.