This extension aims to warn you whenever you visit a production website.
You can add several domains/patterns (Regexp) in order to match the domains on which the custom html/css code will be injected.
The default message is the following :
<div id='warner-prod'>PROD</div>
<style>
#warner-prod{
background:red;
color: white;
padding:1rem;
}
</style>
You can chagne it by clicking the extension icon, then opn the right menu and select "open settings".