user-selectable light and dark themes

cookies, template rendering with different CSS files via default or
request param or cookie, etc.

tests/instance/pages/mdash-or-triple-dash.md

@@ -0,0 +1,3 @@
+Test page
+
+word --- word

tests/test_pages.py

@@ -1,5 +1,7 @@
 """Unit test helper methods."""
-from incorporealcms.pages import generate_parent_navs, is_file_path_actually_dir_path, resolve_page_file
+from werkzeug.http import dump_cookie
+
+from incorporealcms.pages import generate_parent_navs, is_file_path_actually_dir_path, render, resolve_page_file
 
 
 def test_resolve_page_file_dir_to_index():
@@ -74,3 +76,25 @@ def test_is_file_path_actually_dir_path_nonsense_dir_is_no(app):
     """Test that a directory request is a directory request even if the dir doesn't exist."""
     with app.app_context():
         assert not is_file_path_actually_dir_path('/antphnathpnthapnthsnthax/')
+
+
+def test_render_with_user_dark_theme(app):
+    """Test that a request with the dark theme selected renders the dark theme."""
+    cookie = dump_cookie("user-style", 'dark')
+    with app.test_request_context(headers={'COOKIE': cookie}):
+        assert b'dark.css' in render('base.html').data
+        assert b'light.css' not in render('base.html').data
+
+
+def test_render_with_user_light_theme(app):
+    """Test that a request with the light theme selected renders the light theme."""
+    with app.test_request_context():
+        assert b'light.css' in render('base.html').data
+        assert b'dark.css' not in render('base.html').data
+
+
+def test_render_with_no_user_theme(app):
+    """Test that a request with no theme set renders the light theme."""
+    with app.test_request_context():
+        assert b'light.css' in render('base.html').data
+        assert b'dark.css' not in render('base.html').data