wip dark theme support

i want squirrel to use whatever theme the system is set to. currently it’s forced light mode but dark mode is something i’d be crucified for not adding.

unfortunately, I in my infinite wisdom didn’t have the foresight to have consistent colors across even the light theme itself. there’s just random #aaa, #666, etc colors everywhere where i went with what looked good. so work needs to be done to standardize the colors with css variables. then just modify the variables for dark theme accordingly.

yes, i’m aware this is standard practice.