readme update, minor style fixes

src/routes/+layout.svelte

@@ -31,7 +31,7 @@
   <link rel="icon" href={favicon} />
 </svelte:head>
 
-<div class="vbox h-screen min-h-screen box-border overflow-hidden relative">
+<div class="app-container vbox h-screen min-h-screen box-border overflow-hidden relative">
   {@render children()}
 </div>
 

src/style/app.sass

@@ -37,4 +37,8 @@ body
     align-self: stretch
     overflow-y: auto
     min-height: 0
-    flex: 1
+    flex: 1
+
+.app-container
+    max-width: 1200px;
+    margin: 0 auto;