Add JavaScript in a Swift program

Using Swift
1

Hello,
I would like to create a Safari extension. I would like a script (containing a few lines of code to add a style tag to the head) to be executed on website pages.
To do that, I have found “JavaScriptCore” framework, but I I fail to use it…
Thank you by advance for your help!

2

You can use Javascript. This project may help you. GitHub - tourcoder/DTMG: [Safari Extension] Access target links on Google search result page directly

3

Thanks, but I don’t see how to execute javascript on website pages... And is it for MacOS? I would like to use on iOS…
JavaScriptCore doesn’t work for what I want to do?