set the main wrapper div have layout_wide class

I’ve installed free version of skt-corp theme on my wordpress. the problem is the second class on this HTML tag <div class=”wrapper-main layout_box”> I mean layout_box I want it to be layout_wide. the code that generate this class is located inside header.php I don’t know where is the layout setting that affect my case. I already looked up options.php on theme directory and theme customizer in wordpress panel.
How can I make the code to generate layout_wide for me?