You can add themes at least two ways. The easy way is via Cydia ... since you're talking about winterboard, most likely, you've got Cydia on your iphone. You can search for themes by categories. The other way to add themes to your iphone is via SSH. You'll need to download them from sites like ModMyIfone and then you'll need to drop them into the directory /var/mobile/Library/Summerboard/Themes I believe. Hope this helps! Oh, one last bit, if they don't "take", try changing permissions to 755 ...
It's a reference to read-write-executable permissions, I believe. It's something you can do either in Terminal or Transmit, if you're on a Mac. I'm so much less proficient in PC-talk ;-)
its easier to change permissions by installing icommander from cydia. its basically a file explorer with a gui. just click on the files, and you will be able to change permissions easily. oh and you can also put them in library/stash/themes.
so im having same issue, i'm used to using like iphone pc suite but just got a mac and started using ssh. i can't find summerboard/winterboard folder where it should be. am i missing something? i have cydia/icy/winterboard installed on phone and cant even find themes i've installed through cydia