Update codebase.rst with style directory

Author: kpinnipa

docs/contributor/codebase.rst

@@ -24,9 +24,13 @@ This contains test scripts that test our codebase using jest.
 
 Design: ``design/``
 ^^^^^^^^^^^^^^^^^^^
-A directory contains gifs or images that show the design perspective of
+This directory contains gifs or images that show the design perspective of
 our extension.
 
+Style: ``style/``
+^^^^^^^^^^^^^^^^^^^
+This directory contains all of the CSS styling used in the extension.
+
 Documentation: ``docs/``
 ^^^^^^^^^^^^^^^^^^^^^^^^
 This directory contains the Sphinx project for this documentation.