Set html_theme in a simpler way

Author: Perlence

docs/conf.py

@@ -92,12 +92,7 @@
 
 # -- Options for HTML output ---------------------------------------------------
 
-on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
-
-if not on_rtd:
-    import sphinx_rtd_theme
-    html_theme = 'sphinx_rtd_theme'
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+html_theme = 'sphinx_rtd_theme'
 
 # The name for this set of Sphinx documents.  If None, it defaults to
 # "<project> v<release> documentation".