--- a/browser/themes/shared/browser.inc.css
+++ b/browser/themes/shared/browser.inc.css
@@ -125,8 +125,14 @@
 }
 
 :root:not([accessibilitymode]) .accessibility-indicator,
 :root:not([privatebrowsingmode=temporary]) .private-browsing-indicator {
   display: none;
 }
 
 /* End private browsing and accessibility indicators */
+
+/* Force background for datepicker popup to white so themes don't override it */
+#DateTimePickerPanel[active="true"] > .panel-arrowcontainer > .panel-arrowbox,
+#DateTimePickerPanel[active="true"] > .panel-arrowcontainer > .panel-arrowcontent {
+  --arrowpanel-background: #fff;
+}