Sometime certain website offer great WinterBoard theme online and your not find it in Cydia. Let say, you find tons of Winterboard theme at Softpedia. Usually, these Winterboard themes are packed in a zip file. Actually, you can install these theme manually using Disk Aid directly to your iPhone via PC.
Step 1: Download Disk Aid, which is a free file explorer for iPhone. You can check out my post on how to use Disk Aid.
Step 2: Download the Winterboard theme you love and extract the zipped file. For demonstration, I download the Super Mario Theme from Softpedia and extract in the download folder.
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.