Custom Theme Overrides config.json

### Description

After loading a custom skin, all of the skins from the config,json are no longer available

### Steps to reproduce

- Add skins to config.json
- Add a custom skin from url
- Try to use skin from config.json

Describe how what happens differs from what you expected.

i expected the themes from config.json to show up alongside the custom themes

### Version information

/develop
riot-web version: cfd84688c57b-react-0fd06300beca-js-f120533fadb3