I’ve often wondered what the point of Pix is. I know it’s part of the Mint selection of apps. But aren’t forks or offshoots of apps supposed to be at least slightly different? Perhaps adding functionality or appearance the parent app lacks? Pix and Gthumb are IDENTICAL.
Gthumb:
Pix:
It’s possible they forked it in case the Gnome goes in a different direction than they want for gThumb.
I hate libadwaita, and “maybe” the Mint Team does too.
From Linux Mints announcment
The changes which were specific to Pix were cherry picked and re-applied. These include:
A more intuitive default configuration
Support for Cinnamon, MATE, Xfce
Configurable dark-mode
Support for Xapps favorites
I’m not a libadwaita fan either. And I thought that Gnome apps default to that regardless of the distro and/or DE being used. But Gthumb is following the Arc theme I’m using in EOS Cinnamon.
I may be wrong because recently file-roller had an update, and it’s using libadwaita now. Ugly light theme that doesn’t care about my beautiful dark theme. 
So now I’ve switched to using Ark in Dolphin and/or Xarchiver in Nemo.
How do I set xarchiver as default for Nemo? Or do I just have to richt-click>open with?
Nemo Actions. They’re great! Only they show up where they want in the context menu.
Thanks. Not sure what all that means, to be honest. As I already have the ability to right-click>open with xarchiver, I’ll use that for now.
Just went and checked the action. I had it wrong. I’m actually using engrampa, not Xarchiver.
[Nemo Action]
Active=true
Name=Extract Files...
Comment=Extract archives here
Exec=engrampa -h %F
Selection=Any
Extensions=any;
Quote=double
and
[Nemo Action]
Active=true
Name=Compress Files...
Comment=Create a compressed archive
Exec=engrampa -d %F
Selection=Any
Extensions=any;
Quote=double
These create two context menu items. One to extract from an archive in the current directory and close immediately, and another that uses engrampa to compress all your selected files. With the second one, you wouldn’t need to use the “Open With” dropdown.
And like the wiki says: Place the actions in
~/.local/share/nemo/actions/
with the extension .nemo_action.
So I assume I should do the same, just replacing engrampa with xarchiver.
Not really. Each archive manager has different commands, so it may not work the same.
Meaning the below command may not do the same thing in Xarchiver:
engrampa -d
But you can try it and see what it does. Try “xarchiver --help” / “xarchiver -h” to see the commands in a terminal.
FWIW, I use nemo-engrampa from the AUR.
Wait, what?! That exists? And I went through the trouble of creating an action? 
Well, it’s not so bad. At least I know my action won’t break in an update. 
Great! I’ll give that a shot. Thanks.
EDIT: Yes! Worked perfectly. No more fileroller garbage. Thanks, @r0ckhopper !
You’re welcome 
As for the original question, I use Shotwell rather than Pix/Gthumb.
There’s more on libadwaita and avoiding/sidestepping it in the latest Linux Mint Blog.
Nice. I love that they have taken a stance of working on their own forks which can be made to be suitable for all users. Really love the Mint Team overall, especially since they communicate with sound reasoning as opposed to, “This is what we have done; live with it.” — the Apple approach.
Or even Gnome.
I hope the various GTK based DE developers do start working together more closely. Ubuntu Budgie went though a similar exercise to Mint a while ago - https://discourse.ubuntubudgie.org/t/default-applications-review-for-22-10-23-04-and-beyond/5883
Gnome 4+ = Apple Mac
KDE (since the beginning of time) = Windows
Cinnamon = Windows with a Gnome 3 feel
That’s how I see them, at least.
And yes, KDE is the most customisable DE in the world, I know. It’s just in a phase that makes me avoid it right now. That and I’ve really got used to WMs now.