How to add application-specific icons to current theme?

Question

I'm writing a GUI application using Python and Qt. In this application I want to "decorate" dialogs and output data with theme icons like document-open or dialog-error. This is easily retrieved with a statement like:

from PyQt5.QtGui import QIcon
…
… = QIcon.fromTheme("dialog-error")

The exact icon to retrieve is designated by an index into a list, which results in the following code:

DECORATION = [ None, "dialog-error", "document-open", "app-specific" ]
…
     … = QIcon.fromTheme(DECORATION[i])

I'd like "app-specific" to point to some user directory acting like an addition to the current theme, in order to manage theme- and user-icons in the same statement.

Presently, I have given this user-directory the same structure as /usr/share/ with icons/ and applications/ subdirectories. In icons/, there is a hicolor/ directory containing the "size" subdirs and actions/ with my additional icons.

I already added in the application main window initialisation:

    def __init__(self):
      …
        searchPath = QIcon.themeSearchPaths()
        searchPath.insert(0, XDG_DIR)
        QIcon.setThemeSearchPaths(searchPath)
      …

where XDG_DIR is a string containing the absolute path to my icons/ directory.

This doesn't work. I correctly retrieve the "standard" freedesktop.org icons but not mine.

• is the patch valid only in the widget where it is done (and its descendants) and not effective in "independent" widgets like an error dialog?
• should my patch on themeSearchPaths be done before initialising the application (and Qt)?
• should I provide a xxx.theme configuration file? If so, how do I tell the library I only want to add a few icons (not intending to copy everything from a real theme)?

I don't want to create a system-wide theme. I only want the patch to be effective during application execution so that other applications are not polluted.