revert switch to piccolo_theme since that epparently does not (yet) exist in Debian

diff --git a/doc/conf.py b/doc/conf.py
index 926bee8..d96e5c6 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -105,7 +105,7 @@ pygments_style = 'sphinx'
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'piccolo_theme'
+html_theme = 'sphinx_rtd_theme'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
diff --git a/doc/requirements.txt b/doc/requirements.txt
index 82b36e4..cd6467e 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,2 +1 @@
 breathe
-piccolo-theme