Strange black border of image from theme_base

See the FAQ: How to do a minimal reproducible example reprex for beginners for how to illustrate a problem in a way that is reproducible. You just need representative data, as shown below with a built-in dataset.

ggplot(data = mtcars,aes(x = cyl, y = mpg)) + # trailing comma is unnecessary
  geom_point() +
  xlab("Plant species richness") + # generally better to put theme last
  # theme_base() # there is no theme_base in the ggplot2 package
  theme_classic() # this is the theme that reproduces the problem

Try theme_minimal, which eliminates the border. Come back with a separate question if you also want "How to eliminate grid lines from ggplot?"