Finding a theme

Finding a theme is daunting: there are just so many out there. They all have flowery language bigging themselves up but when you install one, they very rarely look like the demo did.
I’ve now looked at so many that me eyes are square and although what I want is very simple I can’t find it.

I’m searching for a theme that has a small menu at the top that does not move when you scroll down.
It’d be fluid and available on all mobile devices.
It would be able to take a shopping cart.

I don’t think those requirements are to complex, yet I’ve looked for weeks and can’t find one like it. Does anyone know of one?
I’d be forever grateful for the help.