[web] introduce preferences settings in account settings

Summary:
This diff introduces the new apperance setting in the "My account" settings screen. This diff is just introducing the new elements to access this interface where we can easily toggle and test the new light mode

Please note that 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

Depends on D9194

Test Plan:
Please see the screenshot below:

Reviewers: atul, inka, rohan

Reviewed By: rohan

Subscribers: ted, ashoat, tomek

Differential Revision: https://phab.comm.dev/D9195