[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