use thematic background-color on 404 page and fix minor layout issues

customize.dist/src/less2/pages/page-404.less

@@ -3,14 +3,13 @@
 
 html, body {
     .font_main();
+    height: 100%;
     margin: 0px;
     padding: 0px;
-    background-color: @cp_static-bg; // FIXME doesn't get applied
+    background-color: @cp_static-bg !important;
     color: @cryptpad_text_col;
     font-family: "IBM Plex Mono";
     #cp-main {
-
-        height: 100vh;
         margin: 0px;
         width: 100%;
         padding-top: 5%;