first tell which custom rom you are using. Go to the mods and themes section of that rom. If there is theme of that rom is available you can download it and flash it from recovery. Simple
If your ROM doesn't support themes - you can use the Xposed framework to install CM or AOKP themes. Install the HKtheme module from the framework. Here's the link to Xposed http://xdaforums.com/showthread.php?t=1574401