*.scss files go into a sub directory of the style/ directory. You can place them in either one of the existing directories (e.g. style/ui/) or add your own one, that is entirely up to you. Even if you have just plain CSS, name the file yourFilename.scss, which will cause it to be included during style compilation. This works because SCSS/SASS is a superset of CSS, that is, everything that is CSS is also valid SCSS/SASS.