Currently in Axure you can do this several ways. You can define an “interaction style effect” for a widget, including most style properties. In the INTERACTION pane, you’ll need to click + Add a style effect like MouseOver then Selected Style --and then to get the full list of properties, click + More Style Properties. Set that up the way you want it to look (or choose a preset Widget Style (which you can create in the Manage Widget Styles dialog (from the STYLE pane.) Then, you can dynamically set that widget to Selected or Unselected to apply/unapply this style.
Axure RP9 added more control for stying “input fields” (text fields and text areas) but I’d like to see more. To get more styling than is available, I’d say your best approach would be to make a text field with no fill and no border, and place it in front of a rectangle widget which you can fully style. (…not necessarily “simple” but also not too complicated.)
If you have more than one style you need to apply to text (other than Disabled and mouseover, mousedown) then you can use the SET TO : rich text option in the Set Text action. This allows you to change font, size, weight, color, spacing, alignment, etc.
For more extensive styling or number of styles, I’d recommend creating a dynamic panel with one state per style (recognizing you probably won’t consider this “simple”.) If you have text included, I would recommend separating the text into its own widget and place that in front of the “container” dynamic panel. Then you won’t have duplicated text strings which would require more maintenance.
If you want something beyond this, you’ll need to send an email to firstname.lastname@example.org and try to be as specific as possible in your request. I agree it would be great to have something like a Set Style action to apply a named Widget Style from a style sheet.