How to change Content area Width

Hi

Recently i Purchased One Genesis Child Theme(Prose). I am not Techie Girl. I have some issue with the Content area width. my content area width is too low and is not looking Good.If any Css Expert here can Suggest me how to increase the Content area Width?? I changed in my style.css which is
#content {
float: left;
padding: 10px 0 20px;
width: 420px;
}

I changed it from 420 to 520 but no change. Can Some one help me ??
My Blog URL : https://www.simplemehndidesigns.org/
I can share my login credentials also if you are ready to help. I tried a lot please help.