Hi,
1) First, you have to use migration tool to migrate all content
http://extensions.joomla.org/extensions ... -migrationAnd read this:
http://www.ostraining.com/blog/joomla/sp-upgrade/2) Now when you have new Joomla 2.5 working, install a new template Black & White only for J2.5 (!)
and add your CSS changes from old version. Of course it may take 30 minutes or few hours, because I'm not sure what changes you have made.
Sorry but there are no shortcuts. There is No Template Migration Tool on market! especially when you made custom changes.