DOC: Add theme switcher and default to lightmode. (#414)

* DOC: Add theme switcher and default to lightmode.

* Make linter happy.

* Add workaround for theme versions that don't support mode switcher.

* Add comment about theme version checking in conf.

Author: rossbar

doc/conf.py

@@ -85,9 +85,26 @@
     "show_prev_next": False,
     "navbar_end": ["search-field.html", "navbar-icon-links.html"],
 }
+# NOTE: The following is required for supporting of older sphinx toolchains.
+#       The "theme-switcher" templated should be added directly to navbar_end
+#       above and the following lines removed when the minimum supported
+#       version of pydata_sphinx_theme is 0.9.0
+# Add version switcher for versions of pydata_sphinx_theme that support it
+import packaging
+import pydata_sphinx_theme
+
+if packaging.version.parse(pydata_sphinx_theme.__version__) >= packaging.version.parse(
+    "0.9.0"
+):
+    html_theme_options["navbar_end"].insert(0, "theme-switcher")
+
+
 html_sidebars = {
     "**": [],
 }
+html_context = {
+    "default_mode": "light",
+}
 
 html_title = f"{project} v{version} Manual"
 html_last_updated_fmt = "%b %d, %Y"