[web] Change modal_body colors in log-in-form.css

Summary: Modify modal_body colors and replace outer glow with outline to match the SIWE web designs (https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=7646%3A134916&t=6T70eVfC0cqPSbXJ-1)

Test Plan:
Before:

After:

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6481