1. Don’t edit your theme using the WordPress editor.
This is a quick-and-dirty approach to development, but you can easily destroy your entire site by mistake with this tool. Instead, edit the files on your machine locally and then use FTP to upload the changed files to the server.
2. Moving the CSS will break the theme.
WordPress requires that themes register their CSS in order to render it to the page. Themes use a function (wp_register_style()) to tell WordPress the name of the stylesheet, its location, and to specify whether or not other stylesheets need to be loaded first.
Roots includes some of these definitions in /inc/roots-scripts.php. Two lines, for a specific example:
wp_enqueue_style('roots_style', get_template_directory_uri() . '/css/style.css', false, null);
wp_enqueue_style('roots_bootstrap_style', get_template_directory_uri() . '/css/bootstrap.css', array('roots_style'), null);
The first line registers roots_style and requires that it be style.css in the /css directory. It has no dependencies.
The second line registers roots_bootstrap_style and requires it to be bootstrap.css in the /css directory. It is dependent on roots_styles, so WordPress will add roots_styles first automatically.