I created a user script so it shouldn't be necessairy to modify the original script. You can install it from here: https://openuserjs.org/scripts/DaveBentfield/Visual_distinction_for_owned_Games_on_GOG.com

If you'd rather want to modify the original Barefoot Essentials script, by "to the end" I meant after the last line of the original script (after the closing brackets):
...
// }

// check version, and show changelog if new
storage.get('last_BE_version', function(last_BE_version) {
if (last_BE_version === undefined) last_BE_version = default_prev_version
else if (settings.get('BE-show-changelog') !== false && cmpVersion(last_BE_version, version) < 0) {
popup.show('Changelog')
}
storage.set('last_BE_version', version)
})
})
}

$(window).on('DOMSubtreeModified', function() {
$(".product-tile:has(.product-tile__labels--in-library)").css("opacity", "0.25")
});

Either way your browser needs to support the DOMSubtreeModified event. You can check wether your browser supports it here: https://caniuse.com/#search=DOMSubtreeModified

Thank you for the help. It seems as if my browser (FF 52.9.0 ESR) does not support it.

The DOMSubtreeModified event is used to detect if any changes happened to the current content of the site.
As your browser doesn't support this event you can try to substitute

$(window).on('DOMSubtreeModified', function() {
$(".product-tile:has(.product-tile__labels--in-library)").css("opacity", "0.25")
});

with

$(function() {
$(".product-tile:has(.product-tile__labels--in-library)").css("opacity", "0.25")
});

That way the changes are applied once the site is loaded. But after changing pages in the store you're required to manually refresh the the site for the changes to take effect again. This is due to the way content is loaded dynamically after the redesign. Unfortunantely I didn't have time to dig into the websites code further at the moment to provide a better solution.

Hope this helps.

Yes, this one works. Thank you again :)

This one works with Waterfox 56.2.5 ( 64 bit ) . Thank you .

edit: Can you modify the script to remove these from the front page ?

main banner on the top
curated collections

Great . No more annoying front page banner and currated stuff .

Another request : Can you merge this with the script ?

https://www.gog.com/forum/general/gog_anniversary_design_community_fix/post1

I'd like to see the news section on the top and want to get rid of the highlights .

I'm afraid I won't be able to merge the scripts but the news section and highlights stuff can be done by replacing the function with:

$(function() {
// Change owned opacity
$(".product-tile:has(.product-tile__labels--in-library)").css("opacity", "0.25");

// Hide carousel
$(".big-spot__carousel-container").hide();

// Hide small spots
$(".small-spots__container").hide();

// Move news
$(".news-section-wrapper").insertAfter(".small-spots__container");
// Fix padding due to hidden carousel + small spots
$(".news-section-wrapper").css("padding-top", "60px");

// Hide curated collections
$(".container--two-columns__child--curated-collection").hide();
// Fix columns due to hidden curated collections
$(".container--two-columns__child--discover-games").parent().
removeClass("container--two-columns");
// Fix columns due to hidden curated collections
$(".container--two-columns__child--discover-games").
removeClass("container--two-columns__child--discover-games");
});

Thank you .

Are you planning to do a full-fledged Firefox extension, like there already are for Steam and Reddit, using WebExtensions API?

I would think that their hands are full just doing the scripting work (amid their own busy lives). A whole new web extension might be a bit much to expect.

There are still MANY people who don't have a browser using web extensions... so a userscript is more than sufficient and compatible...

WebExtensions API is compatible with the Chrome's extension API, and since there is already a Chrome's extension, porting it should be less of a hassle than writing an extension from scratch.

Whereas dealing with Greasemonkey just for one substantial script and just in 1 browser is just messy.

And it is not compatible with older FF versions which several (including myself) pefer to use.