2ND Category Template (side scrolling)

I want to add a completely new horizontal scrolling video post portion to my blog, but I want it to still be part of the same blog. I only want this portion to have one category of posts: videos. So far I have altered a horizontal scrolling theme to my liking I just need help incorporating it into my current blog. This new horizontal scrolling theme only has a style.css, header.php, footer.php and index.php.

I have spent a lot of time reading on this topic on wordpress and other blogs and am still quite confused so help would really be appreciated.

This is what I have done so far besides getting the 2nd theme ready:
1) created category page titled category-3.php (3 being my video id category)
2) threw my index.php code from the horizontal scrolling theme into it.

My understanding is I need to link this category-3.php to my style.css, header. php and footer.php and it will work. i could be completely wrong tho.

is this right? and how would I do this?

thank you!