[web] fix colors in calendar history modal

Summary:
This diff fixes the colors in the calendar history modal

Linear task: https://linear.app/comm/issue/ENG-8355/convert-calendary-history-modal-on-web-to-dark-mode

Test Plan:
Please see the screenshots below

Before:

After:

Reviewers: inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12602