fix(list): item content doesn't have background on pan

Ensure the inner container of the list item inherits the background and
color from its wrapping parent

Author: simeonoff

projects/igniteui-angular/src/lib/core/styles/components/list/_list-theme.scss

@@ -417,6 +417,7 @@
         position: relative;
         padding: map-get($list-item-padding, 'comfortable');
         border-radius: --var($theme, 'item-border-radius');
+        background: inherit;
         z-index: 2;
     }