Get DOM element JS

Question

<div id="login-form" class="login-form__email login-form__email--blue">
   ::before

there I have some SVG and I need to change the property to left:60%, but I don't know have I can do this whis js because this element is not in HTML and I don't know how I can call it`s:

<div id="login-form" class="login-form__email login-form__email--blue">
         ::before(element)
    <div class="form-group">
         <input type="email" name="username" class="form-control" id="id_username">
    </div>
</div>

this is my js on this page

 function getCookie(name) {
        const value = `; ${document.cookie}`;
        const parts = value.split(`; ${name}=`);
        if (parts.length === 2){
        return parts.pop().split(';').shift();
        }
    }
     const isArabic = getCookie('lang') === 'Arabic';
     const loginForm = document.getElementById('login-form');
              loginForm.dir = isArabic ? 'rtl' : 'ltr';
             if(isArabic){
               loginForm.dir = isArabic ? 'rtl' : 'ltr';
               loginFofrm.::before????.left = isArabic ? "60%" : "20%"
                 (#login-form::before)
   }