Fixed #35809 -- Set background color for selected rows in the admin's form select widget.

Regression in b47bdb4cd9149ee2a39bf1cc9996a36a940bd7d9.

Thank you Giannis Terzopoulos for the review, and Tom Carrick and Sarah Boyce
for the review.

django/contrib/admin/static/admin/css/forms.css

@@ -169,6 +169,10 @@ form .aligned select + div.help {
     padding-left: 10px;
 }
 
+form .aligned select option:checked {
+    background-color: var(--selected-row);
+}
+
 form .aligned ul li {
     list-style: none;
 }

docs/releases/5.1.2.txt

@@ -16,3 +16,7 @@ Bugfixes
 
 * Fixed a regression in Django 5.1 that caused a crash of ``JSONObject()``
   when using server-side binding with PostgreSQL 16+ (:ticket:`35734`).
+
+* Fixed a regression in Django 5.1 that made selected items in multi-select
+  widgets indistinguishable from non-selected items in the admin dark theme
+  (:ticket:`35809`).

tests/admin_views/tests.py

@@ -6175,6 +6175,65 @@ class SeleniumTests(AdminSeleniumTestCase):
         )
         self.take_screenshot("selectbox-non-collapsible")
 
+    @screenshot_cases(["desktop_size", "mobile_size", "rtl", "dark", "high_contrast"])
+    def test_selectbox_selected_rows(self):
+        from selenium.webdriver import ActionChains
+        from selenium.webdriver.common.by import By
+        from selenium.webdriver.common.keys import Keys
+
+        self.admin_login(
+            username="super", password="secret", login_url=reverse("admin:index")
+        )
+        # Create a new user to ensure that no extra permissions have been set.
+        user = User.objects.create_user(username="new", password="newuser")
+        url = self.live_server_url + reverse("admin:auth_user_change", args=[user.id])
+        self.selenium.get(url)
+
+        # Scroll to the User permissions section.
+        user_permissions = self.selenium.find_element(
+            By.CSS_SELECTOR, "#id_user_permissions_from"
+        )
+        ActionChains(self.selenium).move_to_element(user_permissions).perform()
+        self.take_screenshot("selectbox-available-perms-none-selected")
+
+        # Select multiple permissions from the "Available" list.
+        ct = ContentType.objects.get_for_model(Permission)
+        perms = list(Permission.objects.filter(content_type=ct))
+        for perm in perms:
+            elem = self.selenium.find_element(
+                By.CSS_SELECTOR, f"#id_user_permissions_from option[value='{perm.id}']"
+            )
+            ActionChains(self.selenium).key_down(Keys.CONTROL).click(elem).key_up(
+                Keys.CONTROL
+            ).perform()
+
+        # Move focus to other element.
+        self.selenium.find_element(
+            By.CSS_SELECTOR, "#id_user_permissions_input"
+        ).click()
+        self.take_screenshot("selectbox-available-perms-some-selected")
+
+        # Move permissions to the "Chosen" list, but none is selected yet.
+        self.selenium.find_element(
+            By.CSS_SELECTOR, "#id_user_permissions_add_link"
+        ).click()
+        self.take_screenshot("selectbox-chosen-perms-none-selected")
+
+        # Select some permissions from the "Chosen" list.
+        for perm in [perms[0], perms[-1]]:
+            elem = self.selenium.find_element(
+                By.CSS_SELECTOR, f"#id_user_permissions_to option[value='{perm.id}']"
+            )
+            ActionChains(self.selenium).key_down(Keys.CONTROL).click(elem).key_up(
+                Keys.CONTROL
+            ).perform()
+
+        # Move focus to other element.
+        self.selenium.find_element(
+            By.CSS_SELECTOR, "#id_user_permissions_selected_input"
+        ).click()
+        self.take_screenshot("selectbox-chosen-perms-some-selected")
+
     @screenshot_cases(["desktop_size", "mobile_size", "rtl", "dark", "high_contrast"])
     def test_first_field_focus(self):
         """JavaScript-assisted auto-focus on first usable form field."""