Narit,
There have been many, many changes from Moodle 3.3 to 3.6. I don't have a test site anymore. We generally keep current minus 1 version for support. Bootstrap might still use these variables:
// Tabs
$nav-tabs-border-color: $gray-300 !default;
$nav-tabs-link-active-color: $gray-900 !default;
$nav-tabs-link-active-bg: $gray-200 !default;
This would make the text very dark and a light gray background color.