Ensure we dont overwrite style object injected into render

sandbox/tabs.py

@@ -145,4 +145,5 @@ class BasicApp(App):
             self.mount(example.widget)
 
 
-BasicApp.run(css_path="tabs.scss", watch_css=True, log_path="textual.log")
+app = BasicApp(css_path="tabs.scss", watch_css=True, log_path="textual.log")
+app.run()

sandbox/uber.css

@@ -14,6 +14,11 @@ App.-show-focus *:focus {
   background: darkslateblue;
 }
 
+#child2 {
+  text-style: underline;
+  background: red;
+}
+
 .list-item {
   height: 10;
   color: #12a0;

src/textual/scrollbar.py

@@ -206,17 +206,17 @@ class ScrollBar(Widget):
         yield "position", self.position
 
     def render(self, style: Style) -> RenderableType:
-        style = self.parent.styles
-        style = Style(
+        styles = self.parent.styles
+        scrollbar_style = Style(
             bgcolor=(
-                style.scrollbar_background_hover.rich_color
+                styles.scrollbar_background_hover.rich_color
                 if self.mouse_over
-                else style.scrollbar_background.rich_color
+                else styles.scrollbar_background.rich_color
             ),
             color=(
-                style.scrollbar_color_active.rich_color
+                styles.scrollbar_color_active.rich_color
                 if self.grabbed
-                else style.scrollbar_color.rich_color
+                else styles.scrollbar_color.rich_color
             ),
         )
         return ScrollBarRender(
@@ -224,7 +224,7 @@ class ScrollBar(Widget):
             window_size=self.window_size,
             position=self.position,
             vertical=self.vertical,
-            style=style,
+            style=scrollbar_style,
         )
 
     async def on_event(self, event) -> None: