If you want to modify Asteroid’s design without losing those modifications on every update, there are two proper ways to do it.
1. Do not edit the theme’s main stylesheet, style.css. Instead add your CSS codes on the Custom CSS option of the theme. Asteroid Options -> Custom CSS These Codes will be saved in the WordPress database so you won’t lose it if you update the theme.
2. Use a Child Theme. With a Child Theme, you can edit a separate style.css to your hearts desire. Using a child theme also allows you to add new custom functions in a separate functions.php. You can use this file to add simple shortcodes and complex functions. Asteroid revisions earlier than version 1.0.6 don’t work properly with a child-theme so you better use version 1.0.6 or later.