The browser's theme doesn't change the theme of the forum okay. Yes it's not a bug but just a forum feature i feel concerned about. If you change your browser to night mode it doesn't change the forum's background as it still remains in white background but assuming the themes are enabled, it can change the whole interface of the forum's site. Hope you get it now 
Let me help you with that. There is an extension called Dark Reader. You can download the suitable version for your browser from their official website. Once installed, you can enable it, and change the settings as per your liking. Here, let me show you how the forum looks when I turn it on:
Here are the settings that I'm using:
So, you should download it, enable it, and apply the settings, and you won't hurt your eyes anymore.
