hi, i'm making css styles and web programming daily - to change change shopify look is just css, if you need to modify some functionality its mainly javascript or php - both of them i'm using daily at my work. i've put 7 days to job completion because its hard to say how long it will takes from tat simple description you've provided and it can be done even within one day, but again, it depends on what the required design look and what exactly you mean with modify to suit your needs