How would I modify this WordPress theme?

Hi I have purchased the Imagpress theme and I have been able to modify the structure using Firebug but I cannot workout where to add the modifications needed.
It is home 5
http://themeforest.net/item/imagpress-flat-magazine-theme-/full_screen_preview/6052835
http://s13.postimg.org/40w76767b/imag_alterations.png
I have deleted the top menu in firebug and modified margins etc.
Any help will be really appreciated
How would I modify this WordPress theme?

See original post by cars111