You can use a snippet of javascript to handle this. The JavaScript Snippet widget in the app store is your best bet. Here's a simple example:
Configure the JavaScript Snippet (Data Source) widget with the JS below and be sure to set up a variable.
this.domNode.previousSibling.style = 'color:${Color}'
This code adds the "color" style to the JS snippet's sibling. So in a setup like this:
You get results like this:
or this:
With the EnumClass app store widget you can set an enum value to a certain class, which holds your custom css. May that is what you are looking for?