Alternative box-shadow for dark theme.

[isoos]

I've been looking into the recommendations for shadows in dark mode, and the most prominent one is just to remove them like in this article. However, another article suggested a different solution, and after a bit of experimentation, I think I like that we can have the same style for both modes.

Note: while the dark background is not really visible in dark mode, the additional opaque black shadow does create a slight contrast with the opaque white inset. To me it does feel like it has some kind of natural depth.

I am a bit undecided upon the "up" or "down" effect: in this article the "elevation" of an element is connected with the darkness and narrowness of the shadow. So if we want to keep being consistent, we want to have smaller shadow on hover (as well as darker, but that's already done). However, the current pub.dev implementation increases the shadow on hover, so I've kept it for now, but maybe we should rather go with the different direction?

[isoos]

Note: a staging should have this online soon (alongside with a restore from backup).

/cc @parlough