AppImage hardcoded to use Adwaita light theme

Summary:

AppImage does not allow changing the GTK theme. GTK Inspector shows that the theme is hard coded with GTK_THEME, and sure enough, the Apprun contains the following line:

  export GTK_THEME=Default # This one should be bundled so that it can work on systems without Gtk

Commenting this out fixes the issue.

Steps to reproduce:

• open Inkscape (AppImage v1.5)
• Try changing the theme

What happened?

Nothing; theme always defaults to Adwaita.

What should have happened?

Theme should have changed.

Version info

(Unlike Snap/Flatpak The version doesn't exactly say it, but this is an AppImage)

Inkscape 1.5-dev (932d418, 2025-10-23)

                      Compile  (Run)
    GLib version:     2.80.0
    GTK version:      4.14.5 (4.14.5)
    glibmm version:   2.78.1
    gtkmm version:    4.14.0
    libxml2 version:  2.9.14
    libxslt version:  1.1.39
    Cairo version:    1.18.0 (1.18.0)
    Pango version:    1.52.1 (1.52.1)
    HarfBuzz version: 8.3.0 (8.3.0)
    Poppler version:  24.02.0

    OS version:       Fedora Linux 42 (Workstation Edition)