How to increase the width of the template?

Haven’t used this theme but looking at the demo you could use:

.container {
    width: 1200px;
}

It’s set to 960px and the sidebar is 33.3% so it should resize accordingly.

Beware that some themes also get their content size from functions.php in such code as

if ( ! isset( $content_width ) ) {
	$content_width = 730;

So if such setting is present, with 1200px wide and 2/3 of content width, you should set such value to 800px.

Moreover, if you have full content-width featured images, it would be a good idea to set their max-width to 800px in functions.php with code such as:

add_theme_support( 'post-thumbnails' );
	set_post_thumbnail_size( 150, 150, true ); // Default Thumb
	add_image_size( 'featured-post-image', 800, 600, true ); // Featured Post Image