I helped bug test this fanmade plug-in that gives BrickLink a dark mode appearance. All code credit goes to izzydesigns. Here are her instructions for installation:

I've created a custom website theme that makes dark mode the default across all Bricklink domains and subdomains

https://userstyles.world/style/24509

(To use my site theme, you need to install the browser extension "Stylus", which you can get by clicking here.)