What's the best way to review 3rd party theme code?

[withviz]

I have seen the previous discussion on themes. There are a couple of 3rd party themes that have recently been updated, but before I install them via grav admin I wondered what I should be looking for in the code in terms of

• checking for things that may break my exisitng site
• checking for 3rd party calls (maybe I don't want to use another external cdn)
• even checking for anything malicious!