Applying its styles to the page?

Is there any program that when coming to any website will automatically add any styles or stuff on it?

Let's say that when going down on any site in the head was inserted the following:

<link href="https://fonts.googleapis.com/css?family=Montserrat" rel="stylesheet">
<style>
*{font-family:'Montserrat'!important;}</code></pre></style>