Do you know of any themes built with css grid or flexbox?
Grid is best suited for a few specific use cases (2D obviously, but also things like overlapping elements) while flexbox usually shines in simpler yet common layout requirements. Use grid when you already have the layout structure in mind, and flex when you just want everything to fit. Layout first vs content first.
you can design your own theme from the scratch, recently i have developed an ecommerce website using nop commerce i have successfully convert a website template and create my own theme for nop commerce.