1) Set the default value of the long-running threshold to 2 minutes for warning and 5 minutes for the alert.

2) Use $color-danger for the alert threshold.

web/pgadmin/dashboard/__init__.py

@@ -154,7 +154,7 @@ class DashboardModule(PgAdminModule):
         self.long_running_query_threshold = self.dashboard_preference.register(
             'display', 'long_running_query_threshold',
             gettext('Long running query thresholds'), 'threshold',
-            None, category_label=PREF_LABEL_DISPLAY,
+            '2|5', category_label=PREF_LABEL_DISPLAY,
             help_str=gettext('Set the warning and alert threshold value to '
                              'highlight the long-running queries on the '
                              'dashboard.')

web/pgadmin/static/scss/_backgrid.overrides.scss

@@ -345,7 +345,7 @@ table.backgrid {
   }
 
   tr.alert {
-    background-color: $color-danger-light !important;
+    background-color: $color-danger !important;
   }
 }