Operating system: Manjaro Linux (Kernel 5.4.18-1-MANJARO)
Graphics card: Intel Haswell-ULT
Broken: 2.83 (sub 2), c650ea9a13f9, greasepencil-refactor, 2020-02-11, 18:37
Grease Pencil Object (Strokes) unaffected by Background Light
Exact steps for others to reproduce the error
Open either the General or the 2D Animation default file. Add a grease pencil monkey and a basic mesh, set some background color and change the strength settings once to 0 and once to 1.
The shading of the Suzanne object stays the same. When adding a spotlight and aiming it towards Suzanne it gets lit properly.
The demo video of the grease pencil lighting shows correct black objects where no scene light is hitting them.