icons not rendered as expected

[BartJansseune]

I have loaded CDN for both react, react-jsonschema-form and bootstrap bootstrap.
Functionally all seems to work as expected but the icons for Delete (X) and Add(+) behave strange:
X => background white when not hoovered over, black cross instead of white

• => background white when not hoovered over, blue when hoover over, but in both cases the + is not visible
  Any thoughts?
  Thanks,
  Bart