The link he gave you is a basic introduction to the method for extending Flarum. You should not edit Flarum's theme files directly, for the same reason that editing Turkish into the English core.yml is not recommended: any updates to Flarum will overwrite your changes. To add AdSense, you'll probably want to create an extension ... or wait for somebody else to create an extension that you can use.
We've already had one or two discussions related to AdSense ... more, actually ... so I imagine it is only a matter of time before someone will come up with the necessary extension.
