Apex uses the blurphone.apk so would need a themed version of that to replace it with. You can just download the ROM or theme, unzip the file then go into the system/app folder, pull out the blurphone.apk file and place the file on your sdcard Now go into the system/app folder on your phone and delete or rename your current blurphone.apk file. Then move the new apk file to the system/app folder on your phone, do not try to install it. Once you've moved it change the permission (by long pressing on the apk file using root explorer) to User: read, write; Group: read and Others: read, then just restart your phone. Just be sure to make a backup prior to doing this just to be safe.