What is Responsive design?
Responsive means that the content will adjust to every screen width and you can control how appearance will be through media queries.
You can find the responsive media queries in responsive.css
CAUTION: Do not change main theme files. If you want to add your own css, please enable the Child Theme and put your custom css in style.css.
If you want to modify any other php template, just copy it to your Child theme and modify it as you need.