Applying custom styles to the page?

Question

css

Is there any program that, when visiting any site, will automatically add some styles or other to it?
Let's say that when visiting a site, the following is inserted in the head:

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

Answer 1

Stylish browser add-on

Answer 2

Stylish piro.sakura.ne.jp/xul/_treestyletab.html.en