docs: Update css for sphinx_rtd_theme to latest version, which includes selectors missing from old version.

docs/source/techspecs/poly_manager.rst

@@ -531,6 +531,7 @@ Here is a complete example of how to create a software 3D renderer using **poly_
 Our example renderer will only handle flat and Gouraud-shaded triangles with depth (Z)
 buffering.
 
+
 Types
 ~~~~~
 
@@ -798,6 +799,7 @@ the current depth buffer value, and only store the pixel/depth value if it’s l
 At the end of each iteration, we advance the 1/Z value by the X-delta in preparation for the
 next pixel.
 
+
 draw_triangle_gouraud
 ~~~~~~~~~~~~~~~~~~~~~