theme 2015 column width change with update

The only thing that changed in the styling in the last update is a specification for screen readers: https://themes.trac.ww.wp.xz.cn/changeset?sfp_email=&sfph_mail=&reponame=&new=267652%40twentyfifteen%2F4.0%2Fstyle.css&old=248591%40twentyfifteen%2F3.9%2Fstyle.css

If I understand your goal correctly, you want to make the sidebar on the left narrower and the content on the right wider? You can do that with this:

#sidebar { width: 15%; }
#content { width: 85%;margin-left: 15% }

@media screen and (min-width: 59.6875em) {
	body:before {
		width: calc(50% - 440px);
	}
}

You can of course adjust the values to your liking. The last specification refers to the dividing line between the two areas.