Fixed style handling in case of multiple grouped layers

For themes that group multiple layers like this:

```xml
      <Theme>
          <Identifier>Provincies</Identifier>
          <d:Title>Provincies-grens</d:Title>          
          <Layer>OR.Provincies-1</Layer>
          <Layer>OR.Provincies-2</Layer>
      </Theme>
```

it sometimes appeared as if only the first layer was rendered. However, actually the style from the first layer was also applied to the second.