The CSS HTML is now added with a button just click the "add CSS code" button in the upper left-hand corner of the editor. You must be in HTML mode for the button to add the CSS HTML.
When your members add or modify the CSS in the text box and hit save it updates automatically.
If your members remove any CSS it is necessary to refresh the page to apply the removal of the CSS.
Your members can use HTML mode to add their CSS or if they want to use visual mode in the text editor to keep their css codes separated and easy to find and manage.
Helpful hint for Google Chrome users in visual mode.
First paste your CSS as plaintext into visual mode, highlight and copy the CSS now you can paste it into the CSS divide and save. See image below.