Border for each post

Hallo.
I would like to ask if it is possible to make border for each post.
I know it is possible for a “standard” post by adding this code:
1 .post{
2 border:2px solid;
3 margin:0 0 40px;
4 padding:15px;
5 text-align:justify;
6 }

But where to put this code into file “single_categories.php”?
Thank you for any advice.

http://ww.wp.xz.cn/extend/plugins/single-categories/