add CSS style rules for 404 error page in main.css file.

static/css/main.css

@@ -33,6 +33,22 @@ a:hover {
     text-decoration: underline;
 }
 
+.fe-404 {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    height: 100%;
+}
+
+.fe-404 img {
+    width: 300px;
+}
+
+.fe-404 p {
+    font-size: 20px;
+}
+
 input[type=file],
 input[type=text],
 input[type=password],