estilo.css

:root {
  /* root scope variables */
}

.dark-theme {
  /* local scope variables */
  --styled-select-placeholder-color: #999;
  --styled-select-color: white;
  --styled-select-background-color: #555;
  --styled-select-border-color: black;
  --styled-select-border-width: 3px;
  --styled-select-border-radius: 5px;

  --styled-select-menu-outer-margin: 10px 0 0 0;
  --styled-select-menu-outer-padding: 0;
  --styled-select-menu-outer-background-color: #555;
  --styled-select-menu-outer-border-color: black;
  --styled-select-menu-outer-border-style: solid;
  --styled-select-menu-outer-border-width: 3px;

  --styled-select-option-background-color: #444;

  --styled-select-option-focused-color: #eee;
  --styled-select-option-focused-background-color: #333;

  --styled-select-option-selected-color: #eee;
  --styled-select-option-selected-background-color: #444;
}