dark background-color when pressing button

I try to implement a carousel with buttons to go back and forth. When I press the buttons on my pc there is no problem but once I press them on my phone the background-color turns gray.

image

Neither in my CSS nor in my HTML is a background-color which could interfere…

.prev-screen,
  .next-screen {
    align-self: stretch;
    background: none;
    border: 0;
    margin-top: 40px;
    color: rgba(#000, 0.25);
    cursor: pointer;
    font-size: 24px;
    opacity: 1;
    outline: none;
    padding: 16px;
    transform: scale(1);
    z-index: 1000;

    &:hover,
    &:active {
      color: #000;
      transform-origin: center;
      transform: scale(1.25);
    }

    &:disabled {
      opacity: 0;
    }
  }

What could lead to that black background?