Add example on how to change theme field

source/3/3.1/3.1.9/sddm_configuration.rst

@@ -34,4 +34,11 @@ from the command line. To set this new theme as your default theme for sddm see
 
     sudo update-alternatives  --config sddm-ubuntu-theme 
 
-from the command line. To add a new background for a particular theme copy that image into the theme folder as root. Then in your theme edit the file theme.conf as root and change the line background= with the path to your image.
+from the command line. To add a new background for a particular theme copy that image into the theme folder as root. Then in your theme edit the file theme.conf as root and change the line background= with the path to your image. To edit theme.conf as root run 
+
+.. code:: 
+
+    sudo nano /usr/share/sddm/themes/lubuntu/theme.conf 
+
+
+