Use color variables for canvas background

For people who work on the dark mode, and at night, ability to have a default canvas color reactive to the system theme change plus ability to use variables for that.

Just two colors inputs: default for light and default for dark is already a huge win.

Global setting for Figma canvas could also help when working on the dark mode components. The only caveat could be publishing library with light/dark theme override but I think for the page it should be omitted.