Theme Silesia – Create a widescreen page

The Silesia theme already has a full-width page template similar to what is described in http://premium.wpmudev.org/blog/create-a-full-width-page-for-wordpress-in-5-simple-steps/ It works for pages, not posts, and you can try it out by opening one of your pages, finding the “Page Attributes” box on the right (note that you may need to enable it in the Screen Options tab at the top if you don’t see it at first), then select the “Full-width Page Template, No Sidebar” option under Template, and click the Update button.

That gets you part of the way there because Silesia has a max-width of 980px built in to the theme. You can change that to 1920px using custom CSS like this:

@media (min-width: 1049px) {
	.site-main,
	.site-footer,
	.site-header .site-title,
	.site-header .site-description,
	.site-header .main-navigation,
	.single-attachment .content-area {
		max-width: 1920px;
	}
	.content-area {
		max-width: inherit;
		width: 75%;
	}
	.site-main .widget-area {
		max-width: inherit;
		width: 20%;
	}
	.comments-area {
		max-width: inherit;
	}
}

The @media part at the top makes it so the change only applies to large screens and smaller screens act normally according to how the theme was designed.

This example is a starting point and it is based on one of the sample pages in the Silesia theme demo site http://silesiademo.wordpress.com/about/ since your site is behind a login and I am unable to see it.

You can make further adjustments as needed. For example, if you wanted the width to be larger than 1920px for some reason, then you could change “1920px” to “inherit” or to “100%”

Custom CSS can be added using a plugin or by creating a child theme. The Jetpack plugin is one that is built by the same folks who make WordPress.com and it has a CSS editor built in. Here’s how to install it if you want to try it out:
http://jetpack.me/support/getting-started-with-jetpack/#install-jetpack