customizable nav color

Some of our tools at HubSpot use color-coding to help clarify what environment you're working with (i.e. QA vs. PROD), and we've gotten requests for Singularity to support this too. This PR allows the background color of the nav bar to be overridden in the config YAML.

Example:
```yaml
ui:
    navColor: "#ff0000"
```

/cc @timmfin  @wsorenson