How can I get the color tool to provide me with a dark theme and give me a background color?

I'm new to material design and was wondering how I can get this tool: https://material.io/color/#!/?view.left=0&view.right=0&primary.color=455A64

to give me my primary color (blue grey: #455a64) as a background with a dark theme?

How do you know what background color to pick if it is not in the pallet? Do you just guess one outside material's pallet? (Can it still be called material design?)

(e.g. How was this background #3 decided without the pallet? Am I missing something?)