fix: use OS theme (dark / light) background

uses Django provided CSS variable "--body-bg"

Author: asfaltboy

advanced_filters/static/advanced-filters/advanced-filters.css

@@ -58,7 +58,7 @@
 
 .white-popup {
 	position: relative;
-	background: #FFF;
+	background: var(--body-bg, #FFF);
 	padding: 20px;
 	width: auto;
 	max-width: 800px;
@@ -70,4 +70,4 @@
 
 .select2-container {
     min-width: 166px;
-}
+}