Absolutely not. Never change the
Adsense code. There are enough means of adsense optimization & customizations available to change the colour, background or border to suit your needs. Do whatever you want to do outside the code, never fiddle within the ad or the search code. They know it when you do. The search code has more limitations to colour and placement, but you should adhere to the rules. The code may stop working and violates the TOS.