Backports are supposed to be installed not by downloading individual .deb
files, but by configuring the entire repository in /etc/apt/sources.list
. See the instructions page for details.
The repository is made in such a way that apt will not automatically install backports even if they have a later version, so it can be safely configured once and then used via:
apt-get install -t wheezy-backports nginx
Alternatively, nginx has its own repository containing 1.10.0 for Wheezy.