On Ubuntu Desktop, the icon theme used by Qt is always 'gnome' (instead of the user set one)

<renato> timp, I believe this is not a bug, these icons does not exists in the hicolor icon theme
<renato> timp, the solution is add these icons in the ubuntu-mobile theme or use a alternative icon

I verified that these icons exist in Humanity, the default Ubuntu icon theme. Assuming image://gicon behaves according to the icon naming spec all that's needed is a complete theme.

The default Ubuntu icon theme is ubuntu-mono-dark on the desktop and ubuntu-mobile on Ubuntu Touch.

grep Inherits /usr/share/icons/ubuntu-mono-dark/index.theme
Inherits=Humanity-Dark,gnome,hicolor

It actually works here. Do you have libqt5svg5 installed?
Can you post your test QML code?

With latest packages, only one of the above icons still fails:

QML QQuickImage: Failed to get image from provider: image://gicon/text-x-python

Attached are the same examples, wrapped in a Row.

And yes, libqt5svg5 is there - it's required by qtdeclarative5-ubuntu-ui-toolkit-plugin.

It looks like Qt does not pick up the current theme ("ubuntu-mono-dark") and falls back on "gnome" instead. The output of QIcon::themeNames() is "" for me.

Test program to verify the theme name returned by QIcon::themeName().

To compile and execute:
qmake && make && ./icontest

For reference: http://qt-project.org/doc/qt-5.0/qtgui/qicon.html#themeName

Qt upstream bug https://bugreports.qt-project.org/browse/QTBUG-25718 could be related.

Sound like Ubuntu desktop might get a QPA plugin:
https://blueprints.launchpad.net/appmenu-qt/+spec/qt5-qpa-appmenu

Maybe sane to plug the theme selection in there?

This bug was fixed in the package ubuntu-system-settings - 0.1daily13.06.17-0ubuntu1

---------------
ubuntu-system-settings (0.1daily13.06.17-0ubuntu1) saucy; urgency=low

  [ Ken VanDine ]
  * Dropped the OnlyShowIn=Unity from the desktop file, unity8 doesn't
    show it .
  * about: Use text and unicode to create the Ubuntu logo .

  [ Sebastien Bacher ]
  * setings: set up for translations Use "ubuntu-system-settings" for
    the domain, better to namespace to avoid name conflicts since all
    the translation are installed in the same directory. The po.pro will
    make the translation be compiled and installed on build, there is
    also a "make pot" target to refresh the template. This commit also
    includes a first translation (fr.po) which was used to test that
    things are working correctly. .
  * settings: use the string capitalization style recommended by design
    Set some strings, like the vendor or model inforations, as not
    translatables since those should be local specific.
  * settings: make the plugin names translatable.
  * settings: update some icons with copies from design, we will replace
    those by theme icons once they are polished and included in the
    theme use a standard icon for the airplane-mode since it's already
    in theme .
  * settings: set the theme to ubuntu-mobile, until lp#1098578 is fixed,
    (toolkit not respecting the theme in use), recommends ubuntu-mobile-
    icons as well since we use icons shipped there. (LP: #1098578)

  [ Ubuntu daily release ]
  * Automatic snapshot from revision 56
 -- Ubuntu daily release <email address hidden> Mon, 17 Jun 2013 07:01:58 +0000

This should have been fixed in Qt 5.1: https://codereview.qt-project.org/49387.

Is that still an issue with 5.1/5.2 packages?

We use the mobile icon theme by default now, see https://bugs.launchpad.net/ubuntu-ui-toolkit/+bug/1284218

I think this bug can be closed.