[dagit] Consistent blue color for nav, search
Summary: I've been using a brighter COBALT-based set of colors for some of the newer UI, but this doesn't match the BLUE-based colors used elsewhere. The BLUE blues are a little deader than the COBALT blues, but for now I think it's better to maintain consistency, and we can revisit colors globally later on.
Test Plan: Toggle on left nav, verify that the blues match. Open search, verify same.
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D6402