Seems that in my case this helped fixing the issue:
gsettings set org.gnome.desktop.interface color-scheme 'prefer-dark'
(still get the warning, though)
even without bypassing the portal with ADW_DISABLE_PORTAL.
@gwillems thanks anyway for the tip. Do you know where is ADW_DISABLE_PORTAL documented? I see it mentioned in many places but didn’t find an authoritative source. I could only find it mentioned in settings: Be less greedy with color-scheme handling (!817) · Merge requests · GNOME / libadwaita · GitLab
thanks