The WebXR button is a simple HTML element which can be styled using CSS, or completely replaced but a custom button of your choice.
A custom button is just an HTML element on your screen. Any Element you wish to be the clickable element that will get people in XR. To use it you create a WebXREnterExitUIButton (WebXREnterExitUIButton | Babylon.js Documentation) and pass it in the customButtons array of the uiOptions of your experience helper (WebXREnterExitUIOptions | Babylon.js Documentation).
This way you are in full control of the button.
If you want to style the existing item, it’s a button element with the class
babylonVRicon. It has a default background image and default colors, and you can change them as you see fit.