diff --git a/phpBB/styles/prosilver/theme/content.css b/phpBB/styles/prosilver/theme/content.css
index 9d87d3217f..9839c2c38c 100644
--- a/phpBB/styles/prosilver/theme/content.css
+++ b/phpBB/styles/prosilver/theme/content.css
@@ -602,6 +602,7 @@ dl.file dd {
 dl.thumbnail img {
 	padding: 3px;
 	border: 1px solid transparent;
+	box-sizing: border-box;
 }
 
 dl.thumbnail dd {