To add google fonts you would need to load a CSS file that calls the fonts and set styles for your theme.

And if you load your fonts directly from Google instead of locally you will also need to add a call to the Google font repository.

Both should be done in the head for better performance.

All this can be done using Concrete5's asset system but you shouldn't modify the theme directly as it is bad practice and can lead to problems in the future.

What you can do is load all that from the file application/bootstrap/app.php

If you read the comments in that file you will see you can do a lot of things from there including overriding and loading assets. You will also find examples showing how to do that.