There's a night mode plugin that puts a lightbulb in the menu bar at the top. I installed that. But, I also have some custom CSS that puts a box with radius corners around each category and topic. The color for that background is specified in the CSS, and doesn't change.
I'm guessing if I didn't have that custom CSS, things would work just fine. I wish there was an option in the persona theme to enable boxes around these things. Having everything on a white canvas with no separation looks messy to me, not minimal.
The setting @julian describes belongs to the browser (chrome/chromium in this case), not NodeBB.
If you're using chrome/chromium, press F12, then F1 to show the options. Check the option and keep the tools open while refreshing the page to ensure the cache is disabled.