Custom theme needs re-design – Developer Support

Hi there,

I’m a designer and have been asked to redesign this website {http://www.outhouseuk.com/) which is built using a custom theme.

Normally I’d work with a developer but he’s unwilling to work with the current theme and wants the site owner to start from scratch with a theme we would purchase from themeforest. They however spent a lot of money on this custom theme and so want me to be able to completely redesign it and make it mobile responsive. This is so they have full licence rights too for using the same design across multiple sites.

As a designer I’m the creative one – I usually provide brands and the layouts for sites. Tech-wise I only have experience with pre-built themes so I’m a bit out of my depth but wanting to learn.

Can anyone tell me will I need to upload a completely new CSS style sheet and if so what would be the best way to start designing that..

Any insights much appreciated.

Thank you and if I can ever return the favour with design support I’d be happy to!