[6.x] Theming: stack background should probably use `content-bg`

[robdekort]

Bug description

When you theme the CP to get a content background like this:

It's currently not being used in the stack selector:

How to reproduce

Change the content-bg

Logs

Environment

Environment
Application Name: Theory Digital
Laravel Version: 12.42.0
PHP Version: 8.4.15
Composer Version: 2.9.2
Environment: local
Debug Mode: ENABLED
URL: theorydigital.test
Maintenance Mode: OFF
Timezone: UTC
Locale: en

Cache
Config: NOT CACHED
Events: NOT CACHED
Routes: NOT CACHED
Views: CACHED

Drivers
Broadcasting: log
Cache: file
Database: sqlite
Logs: stack / single
Mail: smtp
Queue: sync
Session: file

Storage
public/storage: NOT LINKED

Locales
Installed: en
LaravelLang\Actions\Plugin: 1.11.0
LaravelLang\Attributes\Plugin: 2.14.0
LaravelLang\HttpStatuses\Plugin: 3.11.0
LaravelLang\Lang\Plugin: 15.26.3
LaravelLang\MoonShine\Plugin: INCORRECT
LaravelLang\StarterKits\Plugin: 1.6.0
Locales Version: 2.10.0
Protected: en
Publisher Version: 16.7.0

Livewire
Livewire: v3.7.1

Statamic
Addons: 6
Sites: 1
Stache Watcher: Enabled (auto)
Static Caching: Disabled
Version: 6.0.0-alpha.18 PRO

Statamic Addons
marcorieser/statamic-livewire: dev-statamic-6
stillat/relationships: dev-patch-1
studio1902/statamic-peak-browser-appearance: dev-statamic-6
studio1902/statamic-peak-commands: dev-statamic-6
studio1902/statamic-peak-seo: dev-statamic-6
studio1902/statamic-peak-tools: dev-statamic-6

Installation

Fresh statamic/statamic site via CLI

Additional details

No response