fix front page

threejs/lessons/resources/index.css

@@ -5,7 +5,7 @@ body {
 .lesson-main>* {
     margin: 1em 0;
 }
-.background {
+iframe.background {
     position: fixed;
     width: 100vw;
     height: 100vh;

threejs/lessons/resources/lesson.css

@@ -386,7 +386,10 @@ pre.prettyprint, code.prettyprint {
         font-size: 24px;
     }
     iframe {
-        max-width: 95% !important;
+        max-width: 95%;
+    }
+    iframe.background {
+        max-width: 100%;
     }
 }