Default root help samsung galaxy s3

hello everyone,
i'm new to this forum and i don't know anything about roms or root,
i'd like to installthis theme
i watched tutorial on youtube on how to root my s3 using odin, but the problem is when i'm in downlaod mode odin won't find my device,i've tried another usb, another pc and the problem is the same
can someone help me to install this theme?
thank you so much