Layout indicator doesn't adjust its background color to light-themed wallpapers

### What Happened?

The indicator doesn't follow the same theme as the other panel elements.

If you pick a dark-themed wallpaper, you won't notice any problem.

### Steps to Reproduce

1. Select any of the light-themed wallpapers that come with elementary. The one with the mountain covered in snow will work, as it is mostly white.

### Expected Behavior

The indicator should change its background color to a dark color when the wallpaper is light-themed.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_