Never edit any of the theme core files directly! As all your changes will be lost, when updating the theme.
Instead use the included Child Theme to edit or add any theme code. You find a blank child theme within your download, named realty-child.zip. Upload it as you would upload a normal WordPress theme. Go to Appearance > Themes and activate “Realty Child Theme”. You can add your own styles to “style.css” after the “@import” on line 17.
To learn more about how to use a child theme have a look at the official WordPress Codex: http://codex.wordpress.org/Child_Themes
Quick note: functions.php of a child theme, unlike style.css, does not override its counterpart from the parent. Instead, it is loaded in addition to the parent’s functions.php (specifically, it is loaded right before the parent’s file).
Use the child theme to translate the theme into another language: Learn how to translate the theme →