[landing] polish up hover effects

Summary:
Synced with @ted earlier this week, and got feedback from him to add a minor transition to the hover so that the switch in colors was less abrupt.

Depends on D8168

Test Plan:
The effect is subtle but please look at the demo videos

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek, ted

Differential Revision: https://phab.comm.dev/D8169