[web] Separate color in Button

Summary:
Part of ENG-843
Separates color in Button so it can be used somewhere where the color is dynamic (depends on a color of a chat).

Test Plan:

1. Run flow
2. Check if all buttons look correct

Reviewers: tomek, atul, abosh, ashoat

Reviewed By: tomek, atul, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5321