How to use default variable if same variable is not there for dark theme

theming-in-scss.scss

$themes: (
        default: (
                logo: url("../images/brand/logo_vertical.svg"),
                bg: $bg--light,
                text: $text--light,
        ),
        dark: (
                logo: url("../images/brand/logo_vertical--invert.svg"),
                text: $text--dark,
        ),
);

@mixin themed() {
  @each $theme, $map in $themes {
    .theme--#{$theme} & {
      $theme-map: () !global;
      @each $key, $submap in $map {
        $value: map-get(map-get($themes, $theme), '#{$key}');
        $theme-map: map-merge($theme-map, ($key: $value)) !global;
      }
      @content;
      $theme-map: null !global;
    }
  }
}

@function t($key) {
  @return map-get($theme-map, $key);
}

.base {
  @include themed() {
    color: t($text);
    background: t($bg);
  }
}