[web] introduce AppearanceChangeModal

Summary:
This diff introduces the AppearanceChangeModal component which will be used to toggle between light and dark mode for web.

Again I want to reiterate that the designs/UI here is not finalized. @ted still needs to do an official design pass for this, but since we are only showing this internally at the moment, I prioritized getting something "quick and dirty" out so @ted and I can start testing asap with the intention that we will polish this up down the line

Test Plan:
Please see screenshot below

Reviewers: atul, inka, rohan

Reviewed By: atul, rohan

Subscribers: ted, ashoat, tomek

Differential Revision: https://phab.comm.dev/D9203