Qt Flatpak apps don't detect dark mode on GNOME — root cause analysis and workaround

qt-flatpak-dark-mode-gnome.md

Qt Flatpak Apps Don't Detect Dark Mode on GNOME

TL;DR: Qt6 Flatpak apps using the KDE runtime ignore GNOME's dark mode preference. The workaround is three environment variables + Kvantum.

The Problem

If you run Qt/KDE Flatpak apps (qBittorrent, OBS Studio, VLC, etc.) on GNOME with dark mode enabled, they render with a light theme — white backgrounds, light widgets, the works.

This is caused by two bugs in the Qt/KDE stack.

Root Cause

Bug 1: Qt's gtk3 platform theme ignores color-scheme

Since GNOME 42 (2022), dark mode is expressed via:

gsettings set org.gnome.desktop.interface color-scheme 'prefer-dark'

The GTK theme name stays "Adwaita" — it's no longer changed to "Adwaita-dark". GTK4/libadwaita apps read color-scheme natively and handle this fine.

But Qt6's QGtk3Theme (QT_QPA_PLATFORMTHEME=gtk3) only reads gtk-theme-name. It sees "Adwaita", assumes light, and provides a light QPalette. It never checks color-scheme.

Affects: All Qt6 apps on GNOME, not just Flatpak.

References:

• Qt Forum: colorSchemeChanged under Gnome — confirms root cause
• Possibly tracked as QTBUG-141102

Bug 2: KDE's xdgdesktopportal platform theme doesn't provide a dark palette

The KDE Flatpak runtime includes an xdgdesktopportal platform theme (libqxdgdesktopportal.so). It correctly reads the XDG Settings portal:

$ gdbus call --session \
    --dest org.freedesktop.portal.Desktop \
    --object-path /org/freedesktop/portal/desktop \
    --method org.freedesktop.portal.Settings.Read \
    "org.freedesktop.appearance" "color-scheme"
# Returns: (<<uint32 1>>,)  ← 1 = prefer-dark

But it doesn't translate color-scheme = 1 into a dark QPalette. Apps get a light palette regardless.

Affects: All Qt6 Flatpak apps using the KDE runtime on any non-KDE desktop.

References:

• KDE flatpak-platform-plugin
• KDE Bug 422064 (closed as dup, underlying issue unresolved)
• EasyEffects #4475 — same root cause

Workaround

Three environment variables + Kvantum theme engine. Tested on GNOME 49 / Wayland / Fedora 43 / KDE runtime 6.9.

1. Install Kvantum for your KDE runtime versions

# Check which runtimes your apps use
flatpak list --columns=application,runtime | grep org.kde

# Install Kvantum for each branch (skip versions that don't exist)
flatpak install -y flathub org.kde.KStyle.Kvantum//6.9
flatpak install -y flathub org.kde.KStyle.Kvantum//6.10
flatpak install -y flathub org.kde.KStyle.Kvantum//5.15-24.08
# Note: 6.8 build doesn't exist as of Feb 2026

2. Create the Kvantum config

mkdir -p ~/.config/Kvantum
printf '[General]\ntheme=KvGnomeDark\n' > ~/.config/Kvantum/kvantum.kvconfig

KvGnomeDark is a built-in Kvantum theme designed to mimic Adwaita dark on GNOME.

3. Set Flatpak overrides

flatpak override --user \
  --env=QT_STYLE_OVERRIDE=kvantum \
  --env=QT_QPA_PLATFORMTHEME=gtk3 \
  --env=GTK_THEME=Adwaita:dark \
  --filesystem=xdg-config/Kvantum:ro

4. Restart your apps

That's it. Here's what each piece does:

Variable	Purpose
GTK_THEME=Adwaita:dark	Tells Qt's gtk3 platform theme that dark mode is active (works around Bug 1)
QT_QPA_PLATFORMTHEME=gtk3	Qt reads GTK settings → provides dark QPalette for widget backgrounds
QT_STYLE_OVERRIDE=kvantum	Kvantum renders widgets using KvGnomeDark (Adwaita-like dark appearance)
xdg-config/Kvantum:ro	Exposes Kvantum config to the Flatpak sandbox

Limitations

• Not auto-switching: Toggling GNOME light/dark won't update running apps. You'd need to change GTK_THEME and the Kvantum theme manually.
• No Kvantum for all runtimes: Some KDE runtime versions (e.g. 6.8) don't have a Kvantum build. Apps on those runtimes will get the dark palette but default Breeze rendering.
• Approximate match: KvGnomeDark looks like Adwaita but isn't pixel-perfect. For a closer match, try KvLibadwaita.

What Doesn't Work (Saved You the Trouble)

Approach	Result
QT_STYLE_OVERRIDE=adwaita-dark	KDE's KColorScheme overrides the palette — widget chrome goes dark but content areas (tables, dialogs) stay white
QT_QPA_PLATFORMTHEME=xdgdesktopportal	Portal reports dark but palette stays light (Bug 2)
QT_QPA_PLATFORMTHEME=gnome (QGnomePlatform)	Unmaintained for Qt6, can't read dconf inside Flatpak sandbox
kdeglobals with ColorScheme=BreezeDark	KDE platform theme ignores it inside Flatpak
Kvantum with respect_DE=false	Platform theme still overrides the palette
QT_QPA_PLATFORMTHEME=gtk3 alone	Reads gtk-theme-name=Adwaita, assumes light (Bug 1)

Environment

• OS: Fedora 43 (Bluefin) — but this affects any GNOME 42+ system
• Desktop: GNOME 49.2 / Wayland
• Apps tested: qBittorrent 5.1.4, OBS Studio, VLC, Fedora Media Writer
• KDE runtimes: 5.15-24.08, 6.8, 6.9, 6.10