The theme of the World Blood Donor Day in 2018 was "Be there for someone else. Give blood. Share life." This theme highlights the importance of donating blood and the impact it can have on someone else`s life.

Option 1: "Blood connects us all" is not the correct theme for the World Blood Donor Day in 2018. Although blood is a common factor for all humans, the theme focuses more on being there for someone else through blood donation.

Option 3: "Give blood. Give now. Give often" is not the correct theme for the World Blood Donor Day in 2018. This option emphasizes the need for regular blood donation, which is important, but it does not encompass the idea of sharing life and being there for someone else.

Option 4: "Thank you for saving my life" is not the correct theme for the World Blood Donor Day in 2018. While gratitude towards blood donors is important, this option does not capture the essence of the theme which is about actively participating in blood donation to help others.

Therefore, the correct and provided answer is option 2: "Be there for someone else. Give blood. Share life."