add eval() function in replacement of @eval

so to use runtime substitution, you can use the eval() function everywhere in your css:

```
@def MY_BLACK eval('jdramaix.gsssampleapplication.client.GssSampleApplication.black()');
...

.className {
   color: eval('jdramaix.gsssampleapplication.client.GssSampleApplication.black()');
}
```