fixed inclusion of less and sass file

The problem is, that less or sass files which are defined in the theme-config of an extending theme overwrite the ones which are defined in the base theme. 

Due to the fact that in the foreach loop a local index is taken.

Example

*theme.config.php in the base theme.*
```php
'less' => array (
   'base.less' // index is 0
)
```

results in 

```php
$this->less[0] = 'base.less'
```

*theme.config.php in the extending theme*
```php
'less' => array(
 'extending.less' // index is 0 too
)
```

results in 

```php
$this->less[0] = 'extending.less' // instead of $this->less[1] = 'extending.less'
```

Or am I missing a point?