[web] First pass at styling ThreadSettingsModal

Summary:
Started with existing styles (which were borrowed from user-settings-modal.css and worked on styling ThreadSettingsModal so it looks decent.

Matched margin/padding/spacing of LoginForm as much as possible to maintain some consistency.

Before:

After:

Test Plan: Opened ThreadSettingsModal, made some changes, ensured that the changes were made as expected. Did this a few times with thread name, description, color to make sure there weren't any issues.

Reviewers: palys-swm, def-au1t

Reviewed By: palys-swm, def-au1t

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3657