Icons missing in Flatpack apps

Good morning,

since a couple of weeks with Lollypop music player and since today (the app updated this morning) with Foliate I can't see anymore the menu icons (+ - and others) as you can see in the attachments 1 and 2.
I'm using Zorin 15.2 core.
I contacted the guys of Foliate via Github, and this is what one of them told me: "Since it affects GTK's built-in widgets (the spin buttons in the primary menu), and also other Flatpak applications, I'm fairly certain this is not our bug. From the icons which are not missing, it seems that the icon theme used is not Adwaita, but something based on Paper. So this is probably something wrong with the icon theme used by Zorin OS in Flatpak."

So now I'm asking you if you have any suggestion to solve the issue...
Thank you so much
Simone Frignani

Can you install gnome tweak tool

Code:

sudo apt-get install gnome-tweaks

and change your Icon Theme to Adwaita (That developers recommendation) and see if things look normal, again?
If so, then Zorin O.S. may need a quick icon fix.

thanx for the answer!
gnome tweak tool installed, icons changed to Adwaita and... it works: now I can see the icons!
so yep, I guess Zorin needs an icon fix. How is it possible to contact the developers team?

Either PM AZorin or fill out the contact form on the main page of https://zorinos.com

It's impossible to contact them from their site: the contact forum can be used for business enquiries only.
So I wrote a PM.
Thank you again for the help