Update May 2021: Croatia reopened boarders for all EU citizens who are currently in the so-called green list of the European Centre for Disease Prevention and Control. All other passengers need a negative PCR test or rapid antigen test, vaccination certificate or confirming that the holder recovered from the SARS-CoV-2 virus infection or self-isolation is determined for a period of ten (10) days.

Further information can be found here:

https://mup.gov.hr/uzg-covid/english/286212