TL;DR - I installed the adw-gtk3 theme using its instructions and the option to select it doesn’t show up in GNOME Tweaks.
I want to install the adw-gtk3 theme from here and use it on both my system base and layered apps, and my Flatpak apps.
What I’ve tried so far:
I installed GNOME Tweaks as an rpm-ostree layer. Then, I downloaded the tarball as per the instructions in the README, and extracted it to /var/home/.themes. I had to manually create the .themes directory. Then, I installed the flatpak theme modules using the instructions in the README. Finally, I installed the User Themes GNOME extension from Extension Manager. However, the adw-gtk3 and adw-gtk3-dark options do not show up in the drop-down menu like they do on other distros. When I try to set the theme using gsettings it sets the theme to default Adwaita instead, presumably because it did not find the specified theme. I have found that the themes present in Tweaks are located in the /usr/share/themes directory, which is immutable, and I do not wish to make an edit there unless absolutely necessary. The instructions specifically stated that extracting the theme to /usr/share/themes is necessary for applications running as administrator, so I assumed that apps with standard privileges would not need the theme to be there. The instructions also said to extract the theme to ~/.themes in Silverblue instead of ~/.local/share/themes in other distros. I have tried both locations.
Silverblue is an immutable OS and requires special handling that is different than that required in almost every fedora spin. I do not use silverblue nor any of the other immutable versions so have no experience with what you are trying, nor any means nor time to experiment with what you suggest and cannot even offer a guess.
Maybe someone else has tried it, but if not then it seems likely you may have to keep experimenting and eventually figure it out.