How to add an specific command to desktop entry file?

Question

This is my .desktop file :

[Desktop Entry]
Name=Scilab
Exec=/home/bruno/Downloads/scilab-6.1.1/bin/scilab
Terminal=false
Icon=/home/bruno/Downloads/scilab-6.1.1/share/icons/hicolor/48x48/apps/scilab.png
Type=Application

and it works fine, but I would like to initializate with an specific GTK Theme, in terminal I can run:

GTK_THEME=Yaru ./scilab

and this works fine too, but if I try:

[Desktop Entry]
Name=Scilab
Exec=GTK_THEME=Yaru /home/bruno/Downloads/scilab-6.1.1/bin/scilab
Terminal=false
Icon=/home/bruno/Downloads/scilab-6.1.1/share/icons/hicolor/48x48/apps/scilab.png
Type=Application

it doesn't work, any idea of how can i make it work?