[web] [darkmode] unread button

Summary:
theme unread button, no design in web. Generally just theme'ed with our color palette.

Adding ashoat since Atul is OOO today.

Test Plan: should work as before.

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3181