responsive.css

Hi, I would like to modify the mobile css setting (responsive.css, right?). My site (http://kenta-kiyomiya.com/) I guess What I want to do doesn't cause dramatic changes but just want to change the appearance from a mobile like iphone, such as content width (make the content wider to the full screen on both sides), line height (reduce the line heights between content sentences) and so on. In order for that to work, I think I need to modify the responsive.css. I found the previous discussion about this (http://catchthemes.com/support-forum/topic/responsive-css-not-active-in-child-theme/ ), and got some clue. Maybe, I need to add some codes to my child-function.php, and then do something more. Can you tell me what code I should add in my child-function php, and then which part I should change to modify the responsive.css. I hope you get what I want to explain. Thank you