hi
2 ways.
1) FOR SINGLE ACCOUNTS: Click "modify account" under "Account Functions"...choose the account in question and change the theme from the dropdown menu.
2) FOR ALL ACCOUNTS: Click "Addon cPanel Themes" under themes Themes in whm. Install the one you want. Then you go to "Server Setup" and put in what "Default CPanel5 Theme:" you want.
/ditlev