Recommended location for custom style files?

*.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.

JavaScript files go into the js/ directory, look at the existing files and decide where it fits best, don't hesitate to create a new subdirectory if you think that it suits your purpose better. You'll need to manually reference your JavaScript files in your templates, as they're not automatically compiled nor pulled in.