fictioneer-child-theme/css/fictioneer-child-style.css

:root {
  /* General and dark mode. */
}

:root[data-theme=default] {
  /* Only applied if theme is selected. */
}

:root[data-mode=light] {
  /* Only applied in light mode. */
}

:root[data-mode=light][data-theme=default] {
  /* Only applied in light mode and if theme is selected. */
}