theme.scss

/// This function is exactly like `map-get` except it goes deeper.
/// You pass the map as first argument and then an unlimited number of keys to be accessed.
/// If a key cannot be reached, the function aborts and returns `null`.
@function map-deep-get($map, $keys...) {
  @each $key in $keys {
    $map: map-get($map, $key);

    @if (type-of($map) == 'null') { @return $map; }
  }

  @return $map;
}

/// This function is just a wrapper for `map-deep-get`. 
/// What is does is that if `map-deep-get` returns `null` for some reason,
/// then it throws an error. Basically what you wanted I believe.
@function color-palette($keys...) {
  $color: map-deep-get($color-palette, $keys...);
  
  @if not $color {
    @error 'Could not find color for: #{$keys}';
  }
  
  @return $color;
}

$theme-color-1: #ff6600;
$theme-color-2: #000000;

$color-palette: (
  'theme-1': (
    'base': $theme-color-1,
    '10pc': rgba($theme-color-1, 0.1),
    '25pc': rgba($theme-color-1, 0.25)
  ),
  'theme-2': (
    'base': $theme-color-2,
    '10pc': rgba($theme-color-2, 0.1),
    '25pc': rgba($theme-color-2, 0.25)
  ),
);


.side-nav {
  .theme-1 & {
    background-color: color-palette('theme-1', '10pc');
    color: color-palette('theme-1', '25pc');
  }

  .theme-2 & {
    background-color: color-palette('theme-2', '10pc');
    color: color-palette('theme-2', 'FOOBAR');
  }
}