feat: keep contrast on .table-dark

In case we set `$body-bg` to a dark color, we have to keep table contrast relevant.

scss/_variables.scss

@@ -343,7 +343,7 @@ $table-dark-bg:               $gray-900 !default;
 $table-dark-accent-bg:        rgba($white, .05) !default;
 $table-dark-hover-bg:         rgba($white, .075) !default;
 $table-dark-border-color:     lighten($gray-900, 7.5%) !default;
-$table-dark-color:            $body-bg !default;
+$table-dark-color:            $white !default;
 
 $table-striped-order:         odd !default;