💄(admin) allow header color customization

This allows to use custom colors according to
environment.

FIXES #430

src/backend/admin/__init__.py

@@ -0,0 +1 @@
+"""People custom admin site."""

src/backend/admin/apps.py

@@ -0,0 +1,9 @@
+"""Custom Django admin site application configuration."""
+
+from django.contrib.admin.apps import AdminConfig
+
+
+class PeopleAdminConfig(AdminConfig):
+    """Declare our custom Django admin site."""
+
+    default_site = "admin.sites.PeopleAdminSite"

src/backend/admin/sites.py

@@ -0,0 +1,15 @@
+"""Custom Django admin site for the People app."""
+
+from django.conf import settings
+from django.contrib import admin
+
+
+class PeopleAdminSite(admin.AdminSite):
+    """People custom admin site."""
+
+    def each_context(self, request):
+        """Add custom context to the admin site."""
+        return super().each_context(request) | {
+            "ADMIN_HEADER_BACKGROUND": settings.ADMIN_HEADER_BACKGROUND,
+            "ADMIN_HEADER_COLOR": settings.ADMIN_HEADER_COLOR,
+        }

src/backend/admin/templates/admin/base_site.html

@@ -0,0 +1,10 @@
+{% extends "admin/base_site.html" %}
+
+{% block extrastyle %}{{ block.super }}
+<style>
+html[data-theme="light"], :root {
+  {% if ADMIN_HEADER_BACKGROUND %}--header-bg: {{ ADMIN_HEADER_BACKGROUND }};{% endif %}
+  {% if ADMIN_HEADER_COLOR %}--header-color: {{ ADMIN_HEADER_COLOR }};{% endif %}
+}
+</style>
+{% endblock %}