diff --git a/phpBB/styles/prosilver/theme/forms.css b/phpBB/styles/prosilver/theme/forms.css
index 7cc85a7e00..f08a8a9691 100644
--- a/phpBB/styles/prosilver/theme/forms.css
+++ b/phpBB/styles/prosilver/theme/forms.css
@@ -129,6 +129,10 @@ dd select {
 	width: auto;
 }
 
+dd select[multiple] {
+	width: 100%;
+}
+
 dd textarea {
 	width: 85%;
 }