We need more information:
What firmware?
What kind of theme?
Do you have winterboard?
On that note... assuming you have winterboard...
I'm sure there is a way to do it, but wouldn't it just be easier to have big boss host it and install it via cydia? I think to install it through safari you will have to move the unzipped theme you downloaded to the Themes folder for winterboard, then you might have to change the permissions. But even after you do that i'm not sure it will work.
It would help to know if this theme is made to work with winterboard.. there is a special format you have to follow.
how to make your own winterboard theme Tutorial
If the theme follows this format, then it should work once you move it to the Winterboard Theme folder.