Make the preview icon smaller

src/theme.scss

@@ -391,11 +391,13 @@ multiselect {
 // same as settings-avatar
 .app-custom-icon {
     cursor: pointer;
-    width: 128px;
-    height: 128px;
+    width: 64px;
+    height: 64px;
     background-position: center;
     background-size: 100% 100%;
     background-repeat: no-repeat;
+    border: 1px solid gray;
+    border-radius: 3px;
 
     .overlay {
         width: 100%;
@@ -403,6 +405,7 @@ multiselect {
         background-color: rgba(255, 255, 255 ,0.3);
         background-image: url('/img/plus.png');
         background-repeat: no-repeat;
+        background-size: 32px 32px;
         background-position: center;
         transition: all 150ms;
         opacity: 0;