diff --git a/tools/build/build.py b/tools/build/build.py
index 533d5a4a..416b2d4a 100755
--- a/tools/build/build.py
+++ b/tools/build/build.py
@@ -61,6 +61,7 @@ def build_oxjs(downloads=False, geo=False):
     css = read_file(source_path + 'Ox.UI/css/theme.css')
     for theme in themes:
         theme_css = parse_css(css, theme_data[theme])
+        theme_css = theme_css.replace('.png)', '.png?%s)' % version)
         write_file('%sOx.UI/themes/%s/css/theme.css' % (build_path, theme), theme_css)
         write_file('%sOx.UI/themes/%s/css/theme.css' % (dev_path, theme), theme_css)