Add nq-shadow-color as variable

[onmax]

nimiq-ui/packages/nimiq-css/src/css/preflight.css

Line 3 in e86035d

--nq-shadow: 0px 18px 38px rgba(31 35 72 / 0.07), 0px 7px 8.5px rgba(31 35 72 / 0.04), 0px 2px 2.5px rgba(31 35 72 / 0.02);