Setting up responsive content within a responsive theme
If you are using Zen theme, best way(i hope so) to do responsive design is - Zen grids,
First u need to install ruby gems, after you installed u need two more things, as root install sass and compass and ofc. zen-grids

gem install sass 
gem install compass
gem install zen-grids


Then go to your theme folder, and write compass watch now all your css yo need to write in sass folder it will produce css in same files in your css folder. To do blocks responsive, in Layouts folder(ofc if you are using Zen sub theme) u will see responsive-sidebars.scss  or similar, now u need to import @import "zen";// Import Zen Grids with this statement. and write zen-grids code in your responsive-sidebars.scss. Read more about Zen grids - here.
I hope it helps.