Just treat the canvas like any other object you place on the grid and set it to span the entire grid
Category: Grid
.Grid
Grid Properties Background By default the <Page> has a background defined: But you can set a grid background if you want: Columns Rows Placing Objects In The Grid Grid Example 1
Grid background
Setting background colour <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"> <Grid Background="Black">