Temporary Theme

It is possible that I put a temporary theme such as this in my site and modify it?

Such as This?

http://en.hitskin.com/search-a-skin/hogwarts-102210.htm

If yes, How do I do it?

well yes.
first i would take the theme you have now and make that a temp theme.

Admin panel>display>Temporary themes.

click on create a new theme,title it then hit save.
this way if you ever want to go back to that one you can.

Now just go to that hitskin link,type in your forum address.. then preview and if you like it just click install skin.