If you are using Freeway Pro then you can use CSS styles to change the appearance of HTML form items that contain text, such as radio buttons and checkboxes. To do this, select your form item and in the Styles Inspector (the fourth pane of the Inspector) click the + to the right of the section labeled Item:itemName to add or create a new style.
To change the appearance of a button select it in Freeway and go to the Item>Extended menu and in the dialog that appears click New and enter the following:
Where styleName should be replaced for the name of the style you want to use.
Note that Safari will not change the font and size of the text in a button but most other browsers will.
If you are using Express then you can reduce the size of the form item so that only the button or box is visible. With it selected go to the Item Output Settings in the Inspector palette and remove the value in the Text field. You can then place an HTML item alongside the form item, add some text and format that to look any way you like.