The #skillbonus macro is also an option. It only has one parameter, the unique ID of the target skill, because it applies an untyped bonus (which as Mathias noted, stacks with other untyped bonuses).
PreAttr 7000
Code:
doneif (field[gIsEquip].value = 0)
#skillbonus[skAcrobat] += 15
#skillbonus[skStealth] += 15