Add a separate key to make it easy to change the label without changing the
code that relies on the keys. Example: multi-language support.
Rename "txt" to "label", to make it extra clear what it is.
Finally, document the method, say
// RadioButton returns a RadioButton with a label. The key specifies
// the value for the Enum.
func (t *Theme) RadioButton(key, txt string) RadioButton
I don't think this a good name. The widget package is for keeping the state and event
handling, not the particular visual representation. A RadioButton(Group) is just one
particular way of displaying a set of mutually exclusive options.
I suggest widget.Enum or widget.Option. You may have an even better name.