Worked very well! I used siteorigin CSS and this went into the childtheme too.
Another related question:
There is another interesting customisation “Sort posts by blog” (fourth subheading in link above). Adding the snippet shouldn’t be a problem. But what does Mikko mean with:
This will cause the search results to sort by blog ID. In order to get headings between the blogs, watch the $post->blog_id in the search results template and when you notice a change, print out a heading.