[web] introduce light mode to password change modal

Summary:
This diff introduces the new color design system/light mode to the password change modal

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

Here is the WIP spreadsheet @ted and I have been using to communicate what color variables we should use and their associated values
https://docs.google.com/spreadsheets/d/1PwdxT8Ag_4AGzmGSj2wXsR5-go9rsgZvTZycpy3e49k/edit#gid=0

Linear task: https://linear.app/comm/issue/ENG-4804/password-change-modal

Test Plan:
Please see the screenshots below

Light mode:

Dark mode:

Reviewers: atul, rohan, inka

Reviewed By: atul

Subscribers: ashoat, tomek, ted, wyilio

Differential Revision: https://phab.comm.dev/D9688