[web] clean up modal design

Summary: clean up modal , interm. CSS as a whole needs work to get this a lot closer to the intended design.

Test Plan: N/A, make sure modal still works.

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3063