relative url for CSS file

ckan/public/base/css/webassets.yml

@@ -1,18 +1,24 @@
 fuchsia:
+  filters: cssrewrite
   output: base/%(version)s_fuchsia.css
   contents: fuchsia.css
 green:
+  filters: cssrewrite
   output: base/%(version)s_green.css
   contents: green.css
 main:
+  filters: cssrewrite
   output: base/%(version)s_main.css
   contents: main.css
 main-rtl:
+  filters: cssrewrite
   output: base/%(version)s_main-rtl.css
   contents: main-rtl.css
 maroon:
+  filters: cssrewrite
   output: base/%(version)s_maroon.css
   contents: maroon.css
 red:
+  filters: cssrewrite
   output: base/%(version)s_red.css
   contents: red.css