The element on wix is only the mount point to react/Custom element.
You can style the element.
I.E
class ActionBar extends HTMLElement {
constructor() {
super ();
//this.attachShadow({mode: ‘open’}); // sets and returns ‘this.shadowRoot’
this .width = ‘100vw’ ;
this .height = ‘100vh’ ;
this .style.position = ‘fixed’ ;
this .style.left = ‘0’
this .style.bottom = ‘0’ ;
…