docs and comments

src/textual/app.py

@@ -1661,6 +1661,11 @@ async def _on_css_change(self) -> None:
                     self.stylesheet.update(screen)
 
     def render(self) -> RenderResult:
+        """Render method inherited from widget, to render the screen's background.
+
+        May be override to customize background visuals.
+
+        """
         return Blank(self.styles.background)
 
     ExpectType = TypeVar("ExpectType", bound=Widget)

src/textual/screen.py

@@ -265,17 +265,25 @@ def layers(self) -> tuple[str, ...]:
         return (*super().layers, *extras)
 
     def render(self) -> RenderableType:
+        """Render method inherited from widget, used to render the screen's background.
+
+        Returns:
+            Background renderable.
+        """
         background = self.styles.background
         try:
             base_screen = visible_screen_stack.get().pop()
         except IndexError:
             base_screen = None
 
         if base_screen is not None and background.a < 1:
+            # If background is translucent, render a background screen
             return BackgroundScreen(base_screen, background)
 
         if background.is_transparent:
+            # If the background is transparent, defer to App.render
             return self.app.render()
+        # Render a screen of a solid color.
         return Blank(background)
 
     def get_offset(self, widget: Widget) -> Offset: