Always set html_theme in doc configuration.

Lat version of read the doc do not set a html_theme for us.
So we have to always set it.

See readthedocs/readthedocs.org#10638

Author: mgautierfr

docs/conf.py

@@ -24,8 +24,6 @@
 
 # -- General configuration ---------------------------------------------------
 
-on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
-
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
@@ -42,9 +40,7 @@
 # This pattern also affects html_static_path and html_extra_path.
 exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store']
 
-
-if not on_rtd:
-    html_theme = 'sphinx_rtd_theme'
+html_theme = 'sphinx_rtd_theme'
 
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,

docs/requirements.txt

@@ -1,2 +1,3 @@
 breathe
 exhale
+sphinx_rtd_theme