Add Google Tag Manager in amember (or any html codes)

hello guys, I would like to insert a html code in amember, for all pages, how should I do?
in particular I would like to insert the Google tag manager code.

I tried to find here in the forum, but I have not seen anything to help me.
In particular, I need to change the google analytics code (I have to customize it and then I do not just use the amember plugin), because I have to use cross-domain tracking.

Thanks to everyone, even for many of the forum content. I wish you a good day

Go to Control Panel > Setup > Default Theme you can add stuff to the header/footer there, otherwise create a custom theme, activate it and copy in the header template and modify.

Thank you very much, now I'll try and if it works then I'll tell everything.
thank you

I'm sorry Jenolan
I can not find the section that you say. [Control Panel > Setup > Default Theme]
My version is in Italian.

I do not find how to add stuff

You have 'Sample Theme' active. That is there as an example to build your own. Select that and try.

Thank you. I selected the "simple theme", but now I do not know where to act
In te "semple theme section", the only custumizatione thah i can do is to change color of background