If you want to change the appearance of the up and down buttons and substitute the arrows with some text I think that a skinning callback is the answer to your question.
You can check the manual section 22.19 where it explains the skinning of the spin box.
If it is the question you should create a skinning callback that reacts to commands
WIDGET_ITEM_DRAW_BUTTON_L The skinning function should draw the upper button.
WIDGET_ITEM_DRAW_BUTTON_R The skinning function should draw the lower button.
And use text drawing functions inside this "case" statements.