UI: Use theme colors setting for Projectors too

Don't override the theme color settings for the Projector.
If required, the background color of the projector can be changed via

OBSProjector {qproperty-...}

record placed after "OBSQTDisplay {qproperty-...}" section in the theme
file (qss).

UI/window-projector.cpp

@@ -67,7 +67,6 @@ OBSProjector::OBSProjector(QWidget *widget, obs_source_t *source_, int monitor,
 		obs_display_add_draw_callback(GetDisplay(),
 				isMultiview ? OBSRenderMultiview : OBSRender,
 				this);
-		obs_display_set_background_color(GetDisplay(), 0x000000);
 	};
 
 	connect(this, &OBSQTDisplay::DisplayCreated, addDrawCallback);