# 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](https://forum.qt.io/topic/145065/colorschemechanged-under-gnome) — confirms root cause
- Possibly tracked as [QTBUG-141102](https://bugreports.qt.io/browse/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:

```bash
$ 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](https://invent.kde.org/niccolove/flatpak-platform-plugin)
- [KDE Bug 422064](https://bugs.kde.org/show_bug.cgi?id=422064) (closed as dup, underlying issue unresolved)
- [EasyEffects #4475](https://github.com/wwmm/easyeffects/issues/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

```bash
# 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

```bash
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

```bash
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](https://github.com/GabePoel/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