Include css stylesheet in electron app

I created a style.css file. To include it in a .html file I tried :

<link rel="stylesheet" href="https://github.com/karinakozarova/HealthCalc/blob/master/style.css">

and

<base href="/style.css">

and

<href="/style.css">

None of them seem to work. Any ideas how to add CSS to my app via external file will be appreciated.