test(themes): use correct path to local themes

Author: brandyscarney

core/src/themes/native/test/css-variables/index.html

@@ -974,7 +974,7 @@ <h2>Battle Royale Open World Pac-Man</h2>
       function togglePalette(palette) {
         // The path to the directory containing the
         // custom palette files for this test
-        var paletteFilesDir = '/src/themes/test/css-variables/css';
+        var paletteFilesDir = '/src/themes/native/test/css-variables/css';
         let modifier = '';
 
         // The default and dark palettes are official Ionic