fix sizing of logo in header

Signed-off-by: Robin Appelman <robin@icewind.nl>

core/css/header.scss

@@ -114,6 +114,7 @@
 		background-image: url($image-logo);
 		background-repeat: no-repeat;
 		background-position: center center;
+		background-size: contain;
 		width: 62px;
 		height: 34px;
 	}
@@ -538,4 +539,4 @@ nav {
 	li.hidden {
 		display: none;
 	}
-}
\ No newline at end of file
+}