After you choose a location/directory to place your build files in XCode does a quick index of the building blocks: frameworks, plist, strings, etc.

Your app will appear in the Groups & Files column at top, click on it and do then do a Project > Show Inspector. Go to the Styles tab and change the Active Build Style to "Deployment" and make sure that "Zero Link" is unselected. Now open up the "main.nib" file in Interface Builder, select the window style and visible elements in IB's Tools > Show Info palette. I tried to use my existing nib file resaving it as "main" in the proper directory several times but kept getting build errors. My solution was to copy and paste all elements as a group from my nib to the "main" nib in the build file.

While you're in the nib be sure to edit the menubar to reflect the name of your app in the "About..." command. Once your "main" nib has been modified and saved open up the "InfoPlist.strings" and edit the strings to read whatever you want to display in the "About.." box.

Now the only thing left to do is hit the "Build & Run" button. I didn't time the build process but I'm sure it was two or three minutes at most. Once XCode completes the build your theme preview app will auto launch. Bip, bam, boom, you're done!