Use flex to center #message-box

- Resolves #1270
- See details on the issue mentioned.

js/dlg/message-box.css

@@ -3,6 +3,8 @@
   left: 0;
   right: 0;
   bottom: 0;
+  justify-content: center;
+  align-items: center;
   display: flex;
   position: fixed;
   background-color: rgba(0, 0, 0, 0.45);
@@ -17,8 +19,6 @@
 }
 
 #message-box > div {
-  top: 3rem;
-  right: 3rem;
   min-width: 10rem;
   max-width: 50vw;
   min-height: 5rem;