Use in html

useInWebsite.js

const setThemeRelatedToSystem = () => {
  // ... rest of the function's code
}


// Plain JS

document.addEventListener("DOMContentLoad", () => {
  setThemeRelatedToSystem();
})

// Jquery

$(() => {
  setThemeRelatedToSystem();
})

// React JS
/* app.js or index.js */
useEffect(() => {
  setThemeRelatedToSystem();
}, [])