Here's your situation: You've created a Ning Network, and your Design is perfect. You want to use the same design for another Ning Network.
Below, I have listed the steps to accomplish this task. This is the long way. I feel it's the best way. In case anyone brings this up: you could copy/paste ALL CSS (original theme, platform CSS, your theme, etc...) with Firebug. Fact: This is messy, not reliable, and would go way over the character limits in your CSS box. This I would not advise.

To apply the cleanest method, follow these steps:

• From Original Ning Network:
• Go to Manage/Appearance/Basic Theme
  • Locate the original theme you selected.
  • Make note of the theme name.
• Go to Manage/Appearance/All Options
  • Note or record any colors/fonts you have edited, outside of the original theme you selected.
  • I typically flip back and forth between old/new to make sure I have all the same colors.
  • Do be sure you have selected the tab "All Options" when you compare your colors.
  • Make notes if you have uploaded a Network Logo, Background Image, Body Image, Badge Background, and Media Player Image.
• Go to Manage/Appearance/Advanced - Copy/paste all CSS to a text file, and SAVE your text file. You will need this on the new site.
• Go to new Ning Network.
• Go to Manage/Appearance/Basic Theme.
  • Select the same theme you selected on original site. You will receive a warning that your existing CSS will be overwritten. This should be expected.
  • SAVE your changes now, with selected theme.
• Go back to Manage/Appearance/Advanced.
  • Paste all of the CSS from your original (contents of the text file you saved), to your CSS box on new site.
  • SAVE
• Now the hardest part: Go back to Manage/Appearance/All Options
  • Change any colors that were edited, outside of original theme.
  • Upload any images as indicated above.
  • SAVE.

This should do it. This is much easier than it looks. ;-)

I was just telling a fellow NC on the new Ning Creators Network, a lot of us probably take it for granted, how to do this. Hopefully it will help others.