Change the $accordion-icon-color variable default value to
$accordion-button-color
$accordion-button-focus-box-shadow: $btn-focus-box-shadow !default;
$accordion-icon-width: 1.25rem !default;
-$accordion-icon-color: $accordion-color !default;
+$accordion-icon-color: $accordion-button-color !default;
$accordion-icon-active-color: $accordion-button-active-color !default;
$accordion-icon-transition: transform .2s ease-in-out !default;
$accordion-icon-transform: rotate(-180deg) !default;