Hi i like it …but

Please refer the How to section on this page
http://themonic.com/iconic-one/

Steps:
– Install https://wordpress.org/plugins/simple-custom-css/
– Paste this code in appearance -> custom css

.site {
max-width: 1240px;
}

Change 1240 number to higher value as per your requirement.
Pro users get a more advanced CSS customization guide in the member area with direct support too when you need help.