diff --git a/tools/build/build.py b/tools/build/build.py
index b9df07d9..533d5a4a 100755
--- a/tools/build/build.py
+++ b/tools/build/build.py
@@ -52,7 +52,7 @@ def build_oxjs(downloads=False, geo=False):
     # Ox.UI CSS
     css = read_file(source_path + 'Ox.UI/css/Ox.UI.css')
     css = css.replace('$import', '\n'.join([
-        '@import url("../themes/%s/css/theme.css");' % theme for theme in themes
+        '@import url("../themes/%s/css/theme.css?%s");' % (theme, version) for theme in themes
     ]))
     write_file('%sOx.UI/css/Ox.UI.css' % build_path, css)
     write_file('%sOx.UI/css/Ox.UI.css' % dev_path, css)