diff --git a/build-themes.sh b/build-themes.sh
index 3c82631..367a79f 100755
--- a/build-themes.sh
+++ b/build-themes.sh
@@ -4,10 +4,17 @@ set -e
 sass_dir="sass"
 css_dir="data/static/css"
 
-#build default first
-sass --no-source-map "$sass_dir/_default.scss" "$css_dir/style.css"
+if [[ "$1" == "--watch" && -n "$2" ]]; then
+  file="$2"
+  [[ $(basename "$file") = _* ]] && exit 1
+  sass --no-source-map --watch "$file" "$css_dir/theme-$(basename "$file" .scss).css"
+else
+  #build default first
+  sass --no-source-map "$sass_dir/_default.scss" "$css_dir/style.css"
 
-for file in "$sass_dir"/*.scss; do
-  [[ $(basename "$file") = _* ]] && continue
-  sass --no-source-map "$file" "$css_dir/theme-$(basename "$file" .scss).css"
-done
+  rm -rf "$out_dir"/*.css 2>/dev/null
+  for file in "$sass_dir"/*.scss; do
+    [[ $(basename "$file") = _* ]] && continue
+    sass --no-source-map "$file" "$css_dir/theme-$(basename "$file" .scss).css"
+  done
+fi