Use proper Sass variable interpolation to ensure the proper values are compiled and not the literal string of the variable name
$card-border-width: 1px !default;
$card-border-radius: $border-radius !default;
$card-border-color: rgba(0,0,0,.125) !default;
-$card-border-radius-inner: calc($card-border-radius - 1px) !default;
+$card-border-radius-inner: calc(#{$card-border-radius} - #{$card-border-width}) !default;
$card-cap-bg: #f5f5f5 !default;
$card-bg: #fff !default;