Add script code to theme

Hello.

I run a Multisite installation of WP (subfolders). I have the main blog and four other blogs.

I want to add StatCounter script to each of my blogs. StatCounter asks me to add the code right before the closing </body>.

The problem is, I don’t see an option to do that in each blog. Only in the Admin, manaiging all the network, I see the option to edit the theme code.

But this seems to make no sense to me. If I add the code there, by editing the code in the Admin page, how could I add the code to each of the other blogs? This and some other script codes, like the one from Google Analytics, the one from Facebook, etc.

Thank you.

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