Gnome Application Theme

GTK+ ≥ 3.12 allows you to theme specific applications, such as nevolution.

You can use the GTK_THEME=[theme-name] [application] command to run a specific application with a specific theme. For example, you can enter GTK_THEME=Yaru-dark evolution in the terminal to run the evolution of the Yaru dark theme.

If you want to keep this theme permanently in a program,

You should go to /usr/share/applications and find and copy the application’s .desktop file, in this case mine was org.gnome.Evolution.desktop. Then paste it into ~/.local/share/applications and open it with a text editor,

Then after = add env GTK_THEME= look for each line starting with Exec= and put a space after it so the rest of the line comes before it, eg: Exec=env GTK_THEME= Yaru-Dark Evolution -c stream

All done 😄