Get query string in vanilla JavaScript change theme by query parameters

<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css">
    <script src="script.js"></script>
  </head>

  <body>
    <h1>
      change theme by query parameters 
    </h1>
  </body>

</html>

// Code goes here

// Code goes here

 // dedicated method for get query string
 function getQueryString(field, url) {
   let href = url ? url : window.location.href;
   let reg = new RegExp('[?&]' + field + '=([^&#]*)', 'i');
   let string = reg.exec(href);
   return string ? string[1] : null;
 };

 // Theme (?theme=ORANGE_theme)
 function getCurrentTheme() {
   let theme = getQueryString('theme');
   if (theme) {
     if (theme === 'theme_blue') {
       document.querySelector('body').classList.add('theme_blue');
     } else if (theme === 'theme_red') {
       document.querySelector('body').classList.add('theme_red');
     } else if (theme === 'theme_green') {
       document.querySelector('body').classList.add('theme_green');
     }
     console.log('theme', theme);
   } else {
     console.log('No theme ');
   }
 }

 // call method
 getCurrentTheme();

/* Styles go here */