Why has the website changed to that horrid plain white and very light grey ?
It is very hard on the eyes and makes not want to hang around too long to see what is available.

If you're talking about forum color theme, you can change that at the top of the page.

Edit: unless you're talking about the store. Dark theme is only a Galaxy feature, website uses a light theme.

You can also get an extension like Stylus.
https://chrome.google.com/webstore/detail/stylus/clngdbkpkpeebahjckkjfobafhncgmne?hl=en

On gog I use 'Dark gog.com | Improved' and a global style 'Global dark style - change everything to DARK' only on a few websites.

Thanks, I found an add on tool for Firefox that does the trick, my eyes said thank you :)

I'm the same way, bright pages kill me.
I use Night Shift Redux as a dimmer and Dark Reader for most pages as well.