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>
June 8th 19 at 16:42
2 answers
June 8th 19 at 16:44
Stylish addition to the browser
June 8th 19 at 16:46

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