Use the CreateCSS client action to dynamically add CSS class definitions at runtime.  Handy for when your use case needs a class defined by some piece of dynamic data such as an Id.

The Action takes two parameters, a selector and a style definition.  

The selector is a standard CSS selector such as ".myclassname" or "#myid"

The style definition is formatted as a standard CSS style definition such as "color:red; background-color: blue; width:100%; display:flex;"

See https://developer.mozilla.org/en-US/docs/Web/CSS for more CSS info.