This plugin allows you to use uploaded CSS files that can be used across themes without having to edit any theme files or use FTP.
For example, if you wanted to use Font Awesome on your site but your current theme doesn’t include it (or you know you want to use Font Awesome regardless of what theme you’re using), you can simply upload the font-awesome.css (or font-awesome.min.css) file using the WordPress Media Uploader then activate said stylesheet in the Theme-Independent Stylesheets (TISS) settings. That CSS file will then automatically be included on your site no matter what theme you’re using. Even if you switch themes, the CSS file will still be called in your site’s <head> (as long as the theme is coded to properly use wp_head(), which most themes are).