Not able to integrate bootstrap5.3.3 in rails 5

Question

I have downloaded bootstrap 5.3.3 (scss files) and keep them in app/assets/stylesheets/bootstrap_files/ directory

Below is my application.scss file

@import 'bootstrap/functions';
//@import 'bootstrap/variables';

On start of rails server, @import 'bootstrap/functions' works but on uncommenting //@import 'bootstrap/variables'; I got $color: "var(--bs-emphasis-color-rgb)" is not a color for rgba' error.

I tried to downgrade bootstrap to 5.2, 5.1, 5.0 but none works

JS files are present in app/javascript/bootstrap_files/ directory.

I tried to use bootstrap gem but that have some other libraries dependencies , so I cant use that.

I have gem sass-rails & other scss files are already working.

I am using RAILS 5.1.7, RUBY 2.4