Override sphinx-tabs background color (#453)

The sphinx-tabs documentation extension uses a white background 
for the tabs component. This causes readability issues with the theme 
that we have chosen. A custom CSS has been added to override
those components to inherit the overall theme color.

[ committed by @mellis13 ]
[ reviewed by @al-rigazzi ]

doc/_static/custom_tab_style.css

@@ -0,0 +1,7 @@
+.sphinx-tabs-panel {
+    background-color: inherit;
+}
+
+.sphinx-tabs-tab[aria-selected="true"] {
+    background-color: inherit;
+}

doc/changelog.rst

@@ -17,6 +17,17 @@ Development branch
 
 To be released at some future point in time
 
+- Override the sphinx-tabs extension background color
+
+Detailed Notes
+
+- The sphinx-tabs documentation extension uses a white background for the tabs component.
+  A custom CSS for those components to inherit the overall theme color has
+  been added. (SmartSim-PR453_)
+
+.. _SmartSim-PR453: https://github.com/CrayLabs/SmartSim/pull/453
+
+
 
 0.6.0
 -----

doc/conf.py

@@ -100,6 +100,11 @@
     "extra_footer": extra_footer,
 }
 
+# Use a custom style sheet to avoid the sphinx-tabs extension from using
+# white background with dark themes.  If sphinx-tabs updates its
+# static/tabs.css, this may need to be updated.
+html_css_files = ['custom_tab_style.css']
+
 autoclass_content = 'both'
 add_module_names = False