Change webassets configuration for relative urls.

Set the ASSET_URL to be relative to the 'theme/' url, which requires to use
{{ SITEURL }}/{{ ASSET_URL }} in the template and make it works with both
relative and absolute urls.

tests/test_webassets.py

@@ -81,12 +81,16 @@ class TestWebAssets(unittest.TestCase):
     def test_template(self):
         """Look in the output index.html file for the link tag."""
 
-        css_file = 'theme/gen/style.{0}.min.css'.format(self.version)
+        css_file = './theme/gen/style.{0}.min.css'.format(self.version)
         html_files = ['index.html', 'archives.html',
                       'this-is-an-article-with-category.html']
         for f in html_files:
             self.check_link_tag(css_file, os.path.join(self.temp_path, f))
 
+        self.check_link_tag(
+            '.././theme/gen/style.{0}.min.css'.format(self.version),
+            os.path.join(self.temp_path, 'category/misc.html'))
+
     def test_absolute_url(self):
         """Look in the output index.html file for the link tag with abs url."""
 

tests/themes/assets/templates/base.html

@@ -2,6 +2,6 @@
 
 {% block head %}
   {% assets filters="scss,cssmin", output="gen/style.%(version)s.min.css", "css/style.scss" %}
-    <link rel="stylesheet" href="{{ ASSET_URL }}">
+    <link rel="stylesheet" href="{{ SITEURL }}/{{ ASSET_URL }}">
   {% endassets %}
 {% endblock %}