Style all Textual CSS as 'sass'

Textual CSS is better highlighted in SASS code blocks because the SASS parser seems to be more lenient.

docs/blog/posts/spinners-and-pbs-in-textual.md

@@ -396,7 +396,7 @@ Below you can see the code I wrote and a short animation of the app working.
 
 === "CSS"
 
-    ```css
+    ```sass
     Screen {
         align: center middle;
     }