Prices in Croatia are displayed in euros and Kuna, which started in the summer of 2022, but you have been required to pay for things in the new currency since 14 January 2023.
Many shops and small tour operators will take your U.S. dollars if they are desperate for the sale. However, the exchange rate they give you will be horrendous.
If you want to leave a tip, the best way is to round up the bill and tell them you don’t want the change, which would be very appreciated.