do i understood it correct that Aprostope CMS supports Less comiling out of the box ?
see: https://apostrophecms.org/docs/tutorials/getting-started/pushing-assets.html
How to use Sass instead of Less so that we can easily use Bootstrap's Source which is based on Sass, please ?
Apostrophe does compile LESS out of the box but there is no one-button switch to turn on other pre processor flavors like SASS.
The most common route to writing alternative CSS syntax is to run your own asset building process before Apostrophe begins its own.
Common tools for this include Gulp, Webpack, and Grunt.
Linked is a Gist that describes a situation where Gulp.js is compiling SASS to CSS and passing it to the Apostrophe asset pipeline
https://gist.github.com/stuartromanek/972cf94a0f1aedd630e12ca71315f27a