Adding CSS in callbacks of a custom script

I`m trying to add two buttons in a page node using Callback in a custom script. Can we add CSS in the callbacks to change the look and feel of the buttons?
Please find the code mentioned below,

if (callbacks.isEmpty()) {

action = fr.Action.send( new fr.NameCallback(“common.form.EmailorUsername”),

new fr.ConfirmationCallback(fr.ConfirmationCallback.INFORMATION, ["Login","Register"], 0)

).build()

}

What is the login frontend - is it a custom frontend, or AM login hosted?

In the former case, you could pass anything in a text output callback, for example, it’s just a matter of how the custom frontend interprets the content and translate to a rendering. In the latter case, AM login page, you could use a ScriptTextOutput callback where the script manipulates the DOM, but that requires knowing the page structure, and this is not really recommended due to the risk that a product update can change the layout.