Sort articles asc/desc on category page

I’m about to use the Great! theme for my travel blog where I write articles like a travel diary. Therefore I would like to sort the posts in descending order instead of ascending within a specific category.
On my current non-responsive theme i did this with query_posts( $query_string . '&order=DESC' ); in the category.php, but there is no category.php in the Great! theme.
If I use the above code in the index.php, the posts will be sorted but the categories will be ignored.
How could this be acheived, where should I place the code?