Fix minified CSS variable with leading whitespaces

Fixes #378

Author: RV7PR

src/CSS.php

@@ -740,7 +740,7 @@ protected function extractCustomProperties()
         // PHP only supports $this inside anonymous functions since 5.4
         $minifier = $this;
         $this->registerPattern(
-            '/(?<=^|[;}])(--[^:;{}"\'\s]+)\s*:([^;{}]+)/m',
+            '/(?<=^|[;}])\s*(--[^:;{}"\'\s]+)\s*:([^;{}]+)/m',
             function ($match) use ($minifier) {
                 $placeholder = '--custom-'. count($minifier->extracted) . ':0';
                 $minifier->extracted[$placeholder] = $match[1] .':'. trim($match[2]);