[web] Use colors from themes.css in dropdown
Summary:
Some feedback in D8136 was to use colors from themes.css instead of inlining them in SWMansionIcon.
Depends on D8337
Test Plan: Confirmed that both enabled and disabled dropdowns still look the same
Reviewers: atul, ashoat
Reviewed By: atul
Subscribers: tomek, ted
Differential Revision: https://phab.comm.dev/D8338