Increase the size of expander

Closes #12733
(grafted from ea4e1936235b200489557da2212b2135fa67ddd5)

--HG--
branch : 6.0

sao/src/sao.less

@@ -507,12 +507,13 @@ img.icon {
         cursor: pointer;
         > img {
             float: left;
-            margin-right: 5px;
+            margin-right: 1em;
             [dir="rtl"] & {
                 float: right;
-                margin-left: 5px;
+                margin-left: 1em;
             }
-            width: 1em;
+            width: unit(@line-height-base, em);
+            height: unit(@line-height-base, em);
         }
     }
     label {