feat(rtd): provide custom CSS file for overriding in-use Sphinx theme

* `sphinx_rtd_theme` known issue: long lines in tables do not wrap by default
  - readthedocs/sphinx_rtd_theme#117

docs/_static/css/custom.css

@@ -0,0 +1,18 @@
+/*
+    Override styles for in-use Sphinx theme
+*/
+
+/* override table width restrictions */
+.wy-table-responsive table th
+, .wy-table-responsive table td
+{
+    /* !important prevents the common CSS stylesheets from
+       overriding this as on RTD they are loaded after this stylesheet */
+    white-space: normal !important;
+}
+
+.wy-table-responsive
+{
+    overflow: visible !important;
+}
+

docs/conf.py

@@ -149,3 +149,25 @@
         'manual',
     ),
 ]
+
+
+# -- Functions: `setup`, docstring preprocessing, etc. -----------------------
+
+def setup(app):
+    """Prepare the Sphinx application object.
+
+    Used for providing a custom CSS file for override styles.
+
+    Parameters
+    ----------
+    app : object
+        The Sphinx application object.
+
+    Returns
+    -------
+    app : object
+        The Sphinx application object.
+
+    """
+    app.add_stylesheet('css/custom.css')
+    return app