I know this doesn't apply to your needs, however OSX/macOS has the ability to "Invert Display Color" and I use it all the time for precisely the use case you described.
I used this app [2] to read hacker news and im delighted to find they have a web interface as well which has a dark mode
[1] userstyles.org [2] hn.premii.com
Dark themes really are just way easier on the eyes. Combining the Windows 10 dark theme, the black theme in Office 2016, Firefox Developer Edition, plus a bunch of dark themes for web sites yields a pretty good overall experience.