[web] introduce light mode to confirm leave thread modal

Summary:
This diff introduces the new color design system/light mode to the confirm leave thread modal.

For additional context about this new color design system and the new naming convention here is the notion doc:
https://www.notion.so/commapp/New-Color-System-Proposal-ef80b4e4b9ec42949095056161223a42

Linear task: https://linear.app/comm/issue/ENG-5037/confirm-leave-thread-modal

Test Plan:
Please see screenshots below

dark:

light:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10570