Top level style environment variables

Description

Currently, when setting styles for gum through environment variables, it is necessary to set them for every single command separately. It would be simpler to be able to set top level environment variables with style that then propagate to all commands, while also being able to individually being able to set custom ones for specific commands.

Alternatively a command to print out all environment variables with flags for setting colors. Something like: gum generate-theme --foreground="" --background="" --color1="" ....