Injector is a utility that allows you to specify styles for each website. It lets you create a Custom.css file that can be used to apply your own styles to every web page. The styles are applied on a domain basis, meaning that it won't affect other websites, even those on the same domain. You can do that by defining your own domain name in the extension and using that as the domain in the Custom.css file.

For example, let's say I want to create a new extension called Foo that has its own domain name. Foo.com. Then, I can create a Custom.css file with a style named Foo in it.

I can then create a website called Bar.com that includes the extension Foo. And when that happens, any page on Bar.com will have that style applied to it.

The other key thing to note about this extension is that, unlike Custom.css, this one is open source. It is licensed under GNU Affero GPL, which means that you can modify the code and add your own features to it.