Yay that we now have a native way to customize CSS on a per-sheet basis! Boo that the textarea input field is limited to 512 characters.
You can add this bookmarklet to your browser, and click it to extend the maxlength value of the field to whatever you like! It is currently set to increase the value to 2046, but you can edit that as you wish.
Simply add a new bookmark in your browser, paste the below code into the URL section, give it a name, and save. Then if you run out of characters in the field, click the bookmarklet and continue styling!
Name: Qlik Extend CSS Limit URL:
javascript:(function(){var el=document.getElementById("customCss");if(el){el.setAttribute("maxlength","2046");alert("Maxlength changed to 2046!");}else{alert("Textarea with id 'customCss' not found.");}})();
(Note that it should read "javascript:" (javascript[colon]) but the forums seem to change the colon to ":" so make sure you change it back when creating the bookmarklet)