How to have same content with different themes?

Hello! I have a problem.

I have been using my website for quite some time now and it has a lot of articles. But I decided to make a blog as a subsite, where only posts of certain categories will show up in a very nicely formatted manner. So I have activated network, and I have a separate blog site with a theme of my choosing. But this site is presently empty. What should I do to see my old posts, categories, etc. in the new site, such that it would be just like my old site, but with a changed theme?

Once the settings of that other blog are satisfactory, I wish never to touch it at all. I will keep adding posts to my main website, and it will keep updating itself accordingly. New posts can be handled with multipost plugin, as I gather. But I cannot see any option to handle pre-existing data.

I know this question has been asked before, but running two layouts for two completely different purposes parallelly, while keeping common categories and posts, has not exactly been solved, with or without multisite. Often tutorials show up on how to create a static page with a different layout, but it is not a very honest title, in my opinion, because with it, minor variations of the same theme can be made possible, but two totally different themes are never shown, although it is a very popular demand.

Thanks in advance!