Blog posts with no sidebar are centered, not full width

How can I make blog posts use the full width of the page? (Not FULL full-width, just the default 100% with left and right padding, like a new page.)

I found this topic and inserted the code into functions.php for my child theme, and it comes really close to fixing it, but not quite. Here is the code in question for ease of reference:

function avf_blog_style() { ?>
<script>
(function($){
	var blog = $(".template-blog").length;
	if(blog) $(".main_color.container_wrap").removeClass('fullsize');
})(jQuery);
</script>
<?php
}

add_filter('wp_footer', 'avf_blog_style', 10);

What that leaves me with is a blog post that is no longer centered, is using the full amount of available page, and has default left and right margins – but now I have a skinny right sidebar on the post (maybe 50px wide), even though I have the “no sidebar” layout option chosen.

Any ideas on how to make this phantom sidebar go away? I’m not sure where to go from here.