Custom App Launch Behavior Via .desktop Files

Changing how a Linux application launches — forcing dark mode, injecting an environment variable, adding a HiDPI scale flag — doesn't require patching the app itself. Every launchable app on the system has a .desktop file describing how to run it, and it's possible to override that safely without touching the original.

Why Not Edit the System File Directly

Editing /usr/share/applications/*.desktop directly is a bad idea for two reasons: system updates will silently overwrite it, and the change affects every user on the machine. The fix is to copy the file into ~/.local/share/applications/ — the system always prefers a user-local .desktop file over the system-wide one with the same name.

Create the Local Applications Directory

mkdir -p ~/.local/share/applications

Copy the Original .desktop File

Example for GParted:

cp /usr/share/applications/gparted.desktop ~/.local/share/applications/

Edit the exec= Line

Open the copy:

nvim ~/.local/share/applications/gparted.desktop

The Exec= line is the actual command the system runs to launch the app. To force dark mode, prefix it with an environment variable via env:

# Before
Exec=/usr/sbin/gparted %f

# After — force a dark theme
Exec=env GTK_THEME=Adwaita:dark /usr/sbin/gparted %f

Any other program-specific flag can be added the same way — e.g. --force-device-scale-factor=1.5 for HiDPI scaling.

Update the Desktop Database

update-desktop-database ~/.local/share/applications

This makes the system pick up the local override immediately.

Adjusting Privilege Escalation

For tools like GParted that need root, make sure Exec= routes through the correct wrapper for the desktop environment — pkexec is the standard PolicyKit wrapper:

Exec=pkexec /usr/sbin/gparted

Once saved and the database updated, the next launch from the application menu uses the new behavior — dark mode, custom flags, or the adjusted privilege escalation — without ever touching the original system file.