You have been asking quite a few related questions, so it's best you can refer to the documentation:
Function reference • ggplot2 (tidyverse.org)
In this case add this to your code:
theme(plot.subtitle = element_text(hjust = 0.5))
Modify components of a theme — theme • ggplot2 (tidyverse.org)
Theme elements — margin • ggplot2 (tidyverse.org)