doc: add CSS color styles

Add some color styles we can use (via .. rst-class:: style directive) to
add color to rst tables.  Also introduce a centered class instead of
using the deprecated .. centered:: directive.  Update documentation
guidelines to describe these new styles (background colors).

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/developer-guides/doc_guidelines.rst

@@ -522,6 +522,59 @@ Keep the line length for documentation less than 80 characters to make it
 easier for reviewing in GitHub. Long lines due to URL references are an
 allowed exception.
 
+Background Colors
+*****************
+
+We've defined some CSS styles for use as background colors for paragraphs.
+These styles can be applied using the ``.. rst-class`` directive using one of
+these style names.  You can also use the defined ``centered`` style to place the
+text centered within the element, useful for centering text within a table cell
+or column span:
+
+.. rst-class:: bg-acrn-green centered
+
+   \.\. rst-class:: bg-acrn-green centered
+
+.. rst-class:: bg-acrn-lightgreen centered
+
+   \.\. rst-class:: bg-acrn-lightgreen centered
+
+.. rst-class:: bg-acrn-brown centered
+
+   \.\. rst-class:: bg-acrn-brown centered
+
+.. rst-class:: bg-acrn-lightbrown centered
+
+   \.\. rst-class:: bg-acrn-lightbrown centered
+
+.. rst-class:: bg-acrn-blue centered
+
+   \.\. rst-class:: bg-acrn-blue centered
+
+.. rst-class:: bg-acrn-red centered
+
+   \.\. rst-class:: bg-acrn-red centered
+
+.. rst-class:: bg-acrn-gradient centered
+
+   \.\. rst-class:: bg-acrn-gradient centered
+
+.. rst-class:: bg-lightyellow centered
+
+   \.\. rst-class:: bg-lightyellow centered
+
+.. rst-class:: bg-lightgreen centered
+
+   \.\. rst-class:: bg-lightgreen centered
+
+.. rst-class:: bg-lavender centered
+
+   \.\. rst-class:: bg-lavender centered
+
+.. rst-class:: bg-lightgrey centered
+
+   \.\. rst-class:: bg-lightgrey centered
+
 Drawings
 ********