How to set a custom GTK theme for each application

TL;DR

Create a custom desktop entry which launches the program with custom environment variables

The general process is:

1. Create a custom desktop application entry for the app(or copy the default one under /usr/share/applications/<application-name>.desktop) under ~/.local/share/applications/
2. Open the file you just copied/created with your favorite editor (or be a man and open it in vim /s)
3. Locate the line where it says Exec: and then the name of the program.
4. Add env GTK_THEME=<theme name> right after Exec: and put a space after it. ⚠️ Add the name of the theme, not the path to it. (Don’t ask me how much troubleshooting it took me to find out)
5. Kill all instances of the program, then relaunch it(with an application launcher, not from the command line)
6. The program should have the theme specified in it. If it doesn’t it’s because one of these 3 things:
   1. You don’t have the theme specified installed
   2. You misspelled the theme name
   3. You are launching the binary, not the application entry that we just created

Practical Example with VSCodium

1. Copy the desktop entry from /usr/share/applications/vscodium-bin.desktop

   $ cp /usr/share/applications/vscodium-bin.desktop $HOME/.local/share/applications
   

2. Open the file in vim

   $ vim $HOME/.local/share/applications/vscodium-bin.desktop
   

   ➜ It should look something like this:

[Desktop Entry]
Name=VSCodium
Comment=Code Editing. Redefined.
GenericName=Text Editor
Exec=/usr/share/vscodium-bin/bin/codium --unity-launch %F
Icon=vscodium
Type=Application
StartupNotify=true
StartupWMClass=VSCodium
Categories=Utility;Development;IDE;
MimeType=text/plain;inode/directory;
Actions=new-empty-window;
Keywords=vscode;

[Desktop Action new-empty-window]
Name=New Empty Window
Exec=/usr/share/vscodium-bin/bin/codium --new-window %F
Icon=vscodium

1. Edit what’s after Exec: to make it look like this:

[Desktop Entry]
Name=VSCodium
Comment=Code Editing. Redefined.
GenericName=Text Editor
Exec=env GTK_THEME=deepin-dark /usr/share/vscodium-bin/bin/codium --unity-launch %F
Icon=vscodium
Type=Application
StartupNotify=true
StartupWMClass=VSCodium
Categories=Utility;Development;IDE;
MimeType=text/plain;inode/directory;
Actions=new-empty-window;
Keywords=vscode;

[Desktop Action new-empty-window]
Name=New Empty Window
Exec=env GTK_THEME=deepin-dark /usr/share/vscodium-bin/bin/codium --new-window %F
Icon=vscodium

1. Kill all instances of that application, then relaunch it

Now your application should be using the specified GTK theme

Before:

Ugly white border

After:

No more ugly white border, much better cool dark border