Create a shade in the page setting

It is advisable first to create a layer of filling in your diagram.

In your space of page setting, in the layer control, untick all the layers, except the filling layer.

With the help of the « Add a layer » button, create a layer you will call « Shade » for instance.

Then untick this new layer (it added up automatically above the other layers).

With the help of the « selection of a rectangular zone » tool, select all the entities of your filling.

These appear highlighted.

Use the keyboard short cut CTRL + C in order to copy all your entities.

Untick the filling layer, then re-tick the « Shade » layer and use the short cut CTRL+V in order to paste, in the « Shade » layer, your entities contained in the filling layer.

Accede to the properties of your shade layer with the help of the « Properties » button :

In the dialog box appearing, it is possible to modify the filling color of your shade.
Click on the filling color.

Select the color you desire (grey for instance), then click « OK », and ok again.

With the help of your individual selection tool,

click on your grey filling zone (all the elements must still be selected) and, while holding clicked, slightly drag this zone to the bottom and the right for instance).

In your layers control, click on the layer « Shade » and drag it totally to the bottom of the list.

The results...

Before

After