The following opcodes modify FLTK widget appearance:
FLcolor
FLcolor2
FLhide
FLlabel
FLsetAlign
FLsetBox
FLsetColor
FLsetColor2
FLsetFont
FLsetPosition
FLsetSize
FLsetText
FLsetTextColor
FLsetTextSize
FLsetTextType
FLsetVal_i
FLsetVal
FLshow