The most popular is Dark Mode for Wikipedia which uses only a single css style sheet to enable dark mode on Wikipedia. There are several add-on/extension offerings on Firefox which work well. This feature is also available in Microsoft Edge, through the edge://flags/ menu. This is provided by Chrome's development team, so security should not be a concern, but this will render all web pages in dark mode. If you use Chrome, there is also a built-in experimental/beta setting to force dark mode, set through chrome://flags/ with the description 'Force Dark Mode for Web Contents - Automatically render all web contents using a dark theme'.

However, all browser extensions need to be carefully screened for security issues, some inject advertising, and customization may be more difficult than editing a single CSS file. There are browser extensions for popular browsers that offer a Wikipedia-specific dark mode, and browser extensions that allow you to set custom CSS for a given domain, which could be used with the CSS available above.

Firefox dark mode