[web] Update the styles of password modal

Summary:
We don't have the designs but wanted to make this more or less consistent with what we have in other places. Note that modal and input styles do not match the designs from figma e.g. https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1061%3A63258 so the appearance will change in the future.

Depends on D3475

Test Plan: Open modal and check if it looks ok

Reviewers: benschac, atul, def-au1t, ashoat

Reviewed By: benschac, atul, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3476