Had no idea there was a 3rd scheme added. But it's more like an entirely new design rather than just a color scheme. Can't really say I like it. But then again, I'm one of the weirdos who prefer aluminium over carbon :D

Now that is interesting, and they never took the time to try and find a replacement Website Dev, now that is a cheaping out, seems they forgot the old adage "keep the customer happy" not just low-cost games but the entire experience.
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For a long time, the Aluminum look was fine, but as time goes on people change, I am also surprised after all these years GoG has not at least tried to change a little bit. Nowadays I like the darker theme on websites, more but it's to each their own, and there is where giving more choices is better.

Over the past several years or so, more investment seems to have been put into things like Galaxy 2.0 and the like(i.e. things that the higher ups probably think are more worthy of investment and/or will affect the company's bottom line). My guess is this was somewhat influenced by things like GOG going public(stock shares, etc) all those years ago. Also with many open positions @ GOG including a requirement to live in Poland, I expect that might be affecting their search for competent IT/web help somewhat as well.

Yah, more choices are usually better and I have also found I often like the less bright/"gaudy" themes as well.
(re: the purple theme - I like the layout somewhat but the color/tone of it hurts my eyes, so after trying it I switched back to carbon. Still , tis nice to see that they're trying new things finally)

To be honest, adding Dark Mode awareness to websites (where the browser will automatically "follow" the OS setting making web pages darker if the OS Dark Mode is turned on) is far easier than people think and also done in pure CSS with a "Media Query" without needing Javascript or browser extensions:-

/* Light Mode. Used if Dark Mode is disabled on W10-11. Colour #CECECE is GOG's "Aluminium" background. */
@media (prefers-color-scheme: light) {
body {background-color: #CECECE;}
p {color: black;}
}

/* Dark Mode. Used if Dark Mode is enabled on W10-11. Colour #2F2F2F is GOG's "Carbon" footer. */
@media (prefers-color-scheme: dark) {
body {background-color: #2F2F2F;}
p {color: white;}
}

End result. Top is what the web page looks like normally (Aluminium) as well as the default for older OS's, eg, W7 with no Dark Mode, bottom is how the browser displays it if OS Dark Mode is switched on (more like Carbon except it's completely automatic without needing manual theme changes or browser extensions). It's amazing how many websites still don't use it considering it's been supported since Firefox 67 / Chrome 76 (almost 50 versions ago...)

It would seem posting a quoted message is getting harder and harder for me lately.