Upgrade to prestashop 1.6
with the OneClickUpgrade
module, and use the default prestashop 1.6
skin. I've made the upgrade this way from 1.4 to 1.6 without any problem.
The default 1.6 skin is responsive, so your website will work with mobiles, tablets and computers.
Then adapt the default skin to your actual website colors.
Before doing it, backup your database and your files.
I think it is easier to upgrade than to create your own responsive skin.