By default it’s not possible. But you can do this making some changes on the file. Please read the instruction below:
1) Login to the Admin dashboard
2) Navigate Appearance -> editor menu
3) Select header.php file and add below code before <?php wp_head(); ?>.
<link href=”<?php bloginfo( ‘stylesheet_url’ ); ?>” rel=”stylesheet” type=”text/css”>
4) Update the header.php file
5) Open style.css file , and add
#blog {
width:100% !important;
}
6) Update this file