Sets the cursor theme from which the images for cursor should be taken.
If the windowing system supports it, existing cursors created with [ctor@Gdk.Cursor.new_from_name] are updated to reflect the theme change. Custom cursors constructed with [ctor@Gdk.Cursor.new_from_texture] will have to be handled by the application (GTK applications can learn about cursor theme changes by listening for change notification for the corresponding `GtkSetting`).
the name of the cursor theme to use, or null to unset a previously set value
the cursor size to use, or 0 to keep the previous size