apply 'cp' class for unified styles

customize.dist/index.html

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html>
+<html class="cp">
 <head>
     <title data-localization="main_title">Cryptpad: Zero Knowledge, Collaborative Real Time Editing</title>
     <meta content="text/html; charset=utf-8" http-equiv="content-type"/>