Shopify Theme customization

My project is for personal startup use and I'm looking to customize an existing Shopify theme. The customization I need will be primarily focused on the visual elements of the theme. I need some help to create a smooth, aesthetically pleasing design that works with my vision. Ideally I would like to make sure the theme looks good and functions perfectly on desktop, tablet, and mobile devices. I am experienced with Shopify and understand the basics, however I need some expertise to help take my theme to the next level.