Be more flexible with theme for docs

- use default on RTD
- use alabaster if available
- fallback to default
Signed-off-by: Michal Čihař <michal@cihar.com>

docs/conf.py

@@ -15,8 +15,6 @@
 import sys
 import os
 
-on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
-
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
@@ -107,10 +105,14 @@ pygments_style = 'sphinx'
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-if on_rtd:
+if os.environ.get('READTHEDOCS', None) == 'True':
     html_theme = 'default'
 else:
-    html_theme = 'alabaster'
+    try:
+        import alabaster
+        html_theme = 'alabaster'
+    except ImportErrror:
+        html_theme = 'default'
 
 # 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