Calypso uses Sass to build its CSS and compile it to public/style.css. Remember that all styles end up in a single file, so all styles will apply to every page, all the time. Make sure you namespace your styles for the page you are working on.
If you are adding a new Sass file to assets/stylesheets you will need to reference the file in assets/stylesheets/style.scss for it to load. We have three directories to organize the respective files: layout, sections, and shared. If you are adding a new file you are likely adding it to sections.
Component specific stylesheets are referenced in the file assets/stylesheets/_components.scss.
Check our styleguide for more information on how we use Sass.