Use 'Noir' to Force Dark Mode in Safari
Bright websites ruin dark mode. This app fixes that.

Coming across a light website in dark mode feel like shining a light bulb directly into your eyes. Noir is an extension for Safari that automatically adds a dark mode to websites that are otherwise bright.
Modern operating systems, including Apple devices, offer some kind of system wide dark mode that puts light text on dark backgrounds instead of the more-traditional black-on-white. Basically all applications respect the system wide setting at this point, but many websites do not. Noir helps by modifying websites you browse so that they offer light text on a dark background, effectively forcing them to support dark mode.
I've been testing it for a few days and it works really well, even on web applications like Google Docs. Noir is tied to your system settings by default. This means it only applies dark mode to websites when your device is set to dark mode, which is ideal if you switch between the the two regularly, or if your device automatically switches to dark mode.
To get started, you need to purchase and install the application—it costs $2.99 for iOS and $3.99 for Mac (it's a one-time purchase—there's no subscription). The application guides you through opening the settings in Safari and enabling the extension on all websites, which is necessary in order for it to function. After that, when dark mode is enabled, you'll get dark versions of all websites you visit.
For the most part, you can just stop thinking about the application at this point—it will just do its thing in the background. If you want to configure things a little, or if a particular website isn't working well with the change, you can click the Noir icon to change the settings. You can decide whether to disable the extension for the current site. You can also choose a custom theme for the current site. There are four themes offered—Black, the darkest of the four; Dark, the default; Gray, a lighter option with less contract; and Sepia, the lightest theme offered.

It's nice to have a few options, particularly if the default doesn't look great on a particular site. And you can change a few system-wide options by opening the application itself. You can choose any of the four themes as the default, which is nice if you end up preferring one of them.

There's also support for keyboard shortcuts, if you're using a Mac, allowing you to enable and disable the dark theme for the website you're looking at right now or even toggle between themes.
It's a simple application, granted, but it delivers on what it promises. It's easily worth a few bucks if you're a dedicated dark mode user who can't stand bright websites.