It is relatively easy to find the parameter you want to control using JavaScript in the HTML code, by observing which parts of the code change when you change a button attribute. Here is a brief outline of the order of fields for each button:

Fields 0-4: Position and size of button
Fields 4-7: Attributes such as visible, checked, style etc.

Following this are 17 "triples" representing an attribute for each of the 3 button states. If the 3 attributes are different then triple is represented as 3 fields separated by commas. If the 3 attributes are the same for each state, then the triple is represented as a single field beginning with a ^ character.

Triple 0: Bevel Style (up, down, flat etc.)
Triple 1: Bevel Depth
Triple 2: Bevel Shading
Triple 3: Font (0=Arial, 1=Times, 2=Courier, 3=Dialog)
Triple 4: Font Size
Triple 5: Font Style (bold, italic, underline)
Triple 6: Text Alignment (left, center, right, offset)
Triple 7: Text X-offsets
Triple 8: Text Y-offsets
Triple 9: Bitmap Alignment (left, center, right)
Triple 10: Border thickness
Triple 11: Bitmap X-offsets (not used)
Triple 12: Bitmap Y-offsets (not used)

Triple 13: Bitmap filename
Triple 14: Sound filename
Triple 15: Button Text
Triple 16: Button Action

Following this are 4 extra fields:

Field 0: Button link URL
Field 1: Button link target frame
Field 2: Button Name
Field 3: Status line text

Following this are 3 color triples:

Triple 0: Button Color
Triple 1: Text Color
Triple 2: Border Color