Handle dark theme for decorations
Created by: felixfbecker
Closes https://github.com/sourcegraph/sourcegraph/issues/16554 Closes https://github.com/sourcegraph/sourcegraph/issues/16559 Closes https://github.com/codecov/sourcegraph-codecov/issues/61 Closes https://github.com/sourcegraph/sourcegraph/issues/16555 Closes #16594