Following THIS answer:
- create any folder
- create a
/share/themes/ folder inside it (that is
- put your app-specific GTK3 theme inside the latter
- rename the theme to the name of the CURRENTLY ACTIVE THEME.
Use this command to launch the application with the renamed theme instead of the currently active one:
The path after
GTK_DATA_PREFIX= has to be only to the folder that contains
/share/themes/RENAMED_THEME, not to the theme itself
To have that inside a launcher, replace accordingly:
Exec=sh -c 'GTK_DATA_PREFIX=path/to/FOLDER YOUR_APPLICATION'
Exec=env GTK_DATA_PREFIX=path/to/FOLDER YOUR_APPLICATION
Exec=sh -c 'GTK_DATA_PREFIX=path/to/FOLDER /usr/bin/vlc --started-from-file %U'
Exec=env GTK_DATA_PREFIX=path/to/FOLDER /usr/bin/vlc --started-from-file %U
The change can be made in the desktop file in
/usr/share/file/applications or in one copied in
Changing the system theme will reset the per-application theme, until the the folder of the latter is renamed to match the name of the former.