[native] Style SIWE to match designs

Summary:
Designs: https://www.figma.com/file/L675ETKDnGaSwlpZAw4MIC/Mobile-App?node-id=3531%3A51068&t=jPxFWIKYapZrvgr4-0
Design task: https://linear.app/comm/issue/DES-7/designs-for-siwe
Eng task: https://linear.app/comm/issue/ENG-1815/ux-for-signing-a-message-on-native

Test Plan: Visual inspection:

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D5956