As far as I am aware, checkboxes are not able to be styled, possibly a security measure for browsers to avoid security flaws but I can't affirm for sure. (just like one can't fully style the ":active" pseudo class)
In technical terms, what one can do is use pseudo elements (like :after and :before) to style the checkboxes, which is not a particularly simple task especially taking cross-browser into consideration.
You can follow the instructions, add a span checkmark tag to it: https://www.w3schools.com/howto/tryit.asp?filename=tryhow_css_custom_checkbox
Hope it helps!