Glad to see in many reviews the Model 3 has a Dark Mode available on the Tablet UI, that's amazing (seriously that's a big deal, thanks!). Now this website forum needs one.
And the S has had a dark mode for 5 years since it was first available. X has it too. Day to night mode is automatic based on outside brightness. It works quite well.
With respect to the web site, there are Chrome add-ins that can accomplish this for all your browsing. Check the Chrome Web Store and filter for Accessibility and By Google. I think they call it Color Enhancer now. Likely available for other browsers that support add-ins. It is important to some people with low vision or contrast-ish disabilities.
And if what @billstanton said doesn't work, try turning down the brightness on the device you're using.
@TabascoGuy, for most TV shows I try to increase the brightness, but they remain dumb.
I just triple click on my iPad and bam! Dark mode.
@rxlawdude, I have the same issue with a couple of forum members.