Ability to style the Popover overlay

### Provide a general summary of the feature here

Would be very useful to style the Popover overlay the same way as Modal and ModalOverlay.

### 🤔 Expected Behavior?

You should not be able to interact with outside elements while the popover is open.

### 😯 Current Behavior

If there are elements with `zIndex` set, you're able to click them while the popover is still open. That's due to the Popover Overlay not having any `zIndex` which makes it possible to click other elements with `zIndex`.

### 💁 Possible Solution

Either the popover overlay already comes with `zIndex` or an ability to customize the overlay.

### 🔦 Context

We have clickable elements with higher `zIndex` and if you try to close the popover by clicking outside, you can accidentally interact with an element with higher `zIndex` than the overlay.

### 💻 Examples

_No response_

### 🧢 Your Company/Team

_No response_

### 🕷 Tracking Issue

_No response_

Start by locating the Popover overlay implementation and comparing its styling API with Modal and ModalOverlay. Verify that an open popover prevents interaction with outside elements that have a higher z-index, and that the overlay has a default or customizable z-index; confirm the behavior with relevant existing tests if present.

Written by the indexing model from the issue text.