Fix style dictionary warning in output for CSS target

```
While building cpd-common.css, filtered out token references were found; output may be unexpected. Here are the references that are used but not defined in the file
color.theme.bg
color.gray.1400
color.gray.900
color.green.900
color.red.900
color.blue.900
This is caused when combining a filter and `outputReferences`.
```

Caused by https://github.com/vector-im/compound-design-tokens/pull/19