the app should not send emails because of GDPR.
Colors, fonts etc. should be editable.
We use the empire theme.
Most of reviews app allow color, fonts customization, etc. and the app will not send automatically email, if you don't configured them to do so.
Regarding hiding 0 reviews, some apps have this option, other not, but you can achieve that easily with few lines of code (at least is what I did for example using Stamped.io Reviews App)
I am using Stamped.io Reviews app
Disclaimer: I am not affiliated, just personal opinion based on my experience with various store.
You can use Ali Reviews - Product Reviews. We're using it with our e-com businesses. It allows you to edit everything.