Blog: make post listings mobile responsive

src/styles/post-listing.css

@@ -5,10 +5,17 @@
  * The parent <main> must have a class called ".post-listing"
  *
  */
+@media (min-width: 980px) {
     main.post-listing {
         width: 960px;
         margin: auto;
     }
+}
+@media not (min-width: 980px) {
+    main.post-listing {
+        margin: 30px;
+    }
+}
 main.post-listing ul {
     display: flex;
     flex-wrap: wrap;