Step 1: Download Disk Aid form here
http://www.digidna.net/products/diskaid , which is a free file explorer for iPhone
step 2: Download the Winterboard theme you love and extract the zipped file in the download folder.( you can download themes from here
Step 3: Connect your iPhone to your computer via USB cable and launch DiskAid. Then select “Root folder” in the lower-left option. Answer “yes” for the warning. This option will let you browse the whole iPhone file system.
Step 4: On the left pane, go to “Library” and then access “Themes” folder. All Winterboard theme files are placed under this folder. If you have already installed some Winterboard themes on your iPhone, you’ll see those theme folders.
Step 5: Copy the themes you have download, click “Copy Folder to Device” and select the theme folder you just extracted. Click “OK” to confirm and wait for the transfer process to complete.
Step 6: The Winterboard theme is installed on your iPhone. Go back to your iPhone and launch Winterboard. Tap and select “Super Mario Sunshine” to activate your new theme. WinterBoard will mark for you. Press “Home” button and your SpringBoard will restart automatically.