Before and After pseudo classes used with styled-components

Pseudo-selectors in styled-components work just like they do in CSS. (or rather, Sass) Whatever isn't working is likely a problem in your specific code, but that's hard to debug without seeing the actual code!

Here is an example of how to use a simple :after:

const UnicornAfter = styled.div`
  &:after {
    content: " 🦄";
  }
`;

<UnicornAfter>I am a</UnicornAfter> // renders: "I am a 🦄"

If you post your code I'll likely be able to help debug your specific issue!

This will print the triangle at middle of the div.

const LoginBackground = styled.div`
  & {
    width: 30%;
    height: 75%;
    padding: 0.5em;
    background-color: #f8d05d;
    margin: 0 auto;
    position: relative;
  }
  &:after {
    border-right: solid 20px transparent;
    border-left: solid 20px transparent;
    border-top: solid 20px #f8d05d;
    transform: translateX(-50%);
    position: absolute;
    z-index: -1;
    content: "";
    top: 100%;
    left: 50%;
    height: 0;
    width: 0;
  }
`;