Flatpak version can't read system GTK theme

[szija]

What steps will reproduce the problem?

1. Install JOSM from Flathub.
2. Set the theme to ‘GTK+’.
3. Restart the application.

What is the expected result?

The system GTK theme should be used. (Breeze Dark, in my case.)

What happens instead?

JOSM ignores the system GTK theme and uses Adwaita instead.

Please provide any additional information below. Attach a screenshot if possible.

The screenshot below shows the Flatpak version (left) and the version from the Arch Linux repository (right) side by side. Note that only the latter is using Breeze Dark.

Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2025-08-31 14:19:57 +0200 (Sun, 31 Aug 2025)
Revision:19439
Build-Date:2025-09-02 01:30:58
URL:https://josm.openstreetmap.de/svn/trunk

Identification: JOSM/1.5 (19439 hu) Linux Freedesktop SDK 24.08 (Flatpak runtime)
Memory Usage: 320 MB / 1900 MB (114 MB allocated, but free)
Java version: 21.0.9, Flathub, OpenJDK 64-Bit Server VM
Look and Feel: com.sun.java.swing.plaf.gtk.GTKLookAndFeel
Screen: :0.0 1920x1080x[Multi depth]@60Hz (scaling 1.00×1.00) :0.1 1920x1080x[Multi depth]@60Hz (scaling 1.00×1.00)
Maximum Screen Size: 1920×1080
Best cursor sizes: 16×16→16×16, 32×32→32×32
Environment variable LANG: hu_HU.UTF-8
System property file.encoding: UTF-8
System property sun.jnu.encoding: UTF-8
Locale info: hu_HU
Numbers with default locale: 1234567890 -> 1234567890
Desktop environment: KDE
VM arguments: [--module-path=/app/share/openjfx/lib, --add-modules=java.scripting,java.sql,javafx.controls,javafx.media,javafx.swing,javafx.web, -Djosm.restart=true, -Djava.net.useSystemProxies=true, --add-exports=java.base/sun.security.action=ALL-UNNAMED, --add-exports=java.desktop/com.sun.imageio.plugins.jpeg=ALL-UNNAMED, --add-exports=java.desktop/com.sun.imageio.spi=ALL-UNNAMED]

[stoecker]

Report packaging issues to the packager, in this case whoever created the FlatHub stuff.