function setTheme_(theme) {
    var currentUrl = document.location.pathname;

    if (!theme || theme == 'default') {
      localStorage.removeItem('theme');
      document.body.className = '';
      return;
    }

    if (theme) {
      document.body.classList.add(theme);
      localStorage.theme = theme;
    }
  }
Sets the terminal's theme by adding a class
onto the document's body.

@param {String} theme The name of the theme