Use border and toned-down drop-shadow for landing-page cards.

[isoos]

#8307

[isoos]

I've started to push this to staging for better testability.

[sigurdm]

I think the general advice is to not use drop-shadows in dark mode at all.

See https://medium.com/@tundehercules/designing-effective-dark-mode-interfaces-17f38ecea2e9#:~:text=Shadows%20%2CElevation%20and%20Depth

[isoos]

I was planning to keep the two themes conceptually the same, but the argument for not using shadows entirely is a good one. I'll review the other parts of the article and redo this afterwards.