Implement the 'stroke' aesthetic.

[alshan]

See: http://ggplot2.tidyverse.org/reference/geom_point.html

For shapes that have a border (like 21), you can colour the inside and outside separately. Use the stroke aesthetic to modify the width of the border

ggplot(mtcars, aes(wt, mpg)) +
geom_point(shape = 21, colour = "black", fill = "white", size = 5, stroke = 5)