This site does not have a formal dark mode. The theming does not lend itself to having that as an option.
You may want to consider a browser based dark mode extension. I would recommend something open source, subject to code scrutiny, and retaining a separate privacy respecting browser for privacy critical online activity (banking and the suchlike). The use of 2fa aids with resilience to browser based vulnerabilities and exploits, to a large degree. Exercise appropriate caution and your own standards, when using any browser - with or without extensions.
An example of a widely used dark mode extension is Dark Reader. This has extensions for Chrome, Firefox, Edge, Thunderbird, and Safari browsers - though I do not believe the Safari extension is open source.
There are also browsers that natively support dark mode. I use bromite, which is a privacy respecting browser.
This is not an ideal situation, but it is what it is. The experience with the site using a browser enforced dark mode is quite satisfactory. YMMV.