_vendors.scss

/*
Folder structure :

/frontend <-- AngularJS app
    /dist <-- Production ready files
        /assets
        /css
        /js
        index.html
    /src <-- Source files
        /app
        /assets
        /common
        /sass
            /vendors <-- Transformed css to scss partials
            _vendors.scss <-- [CURRENT FILE]
            app.scss <-- Application styles
        index.html
    /vendors <-- Bower default installation folder
gulpfile.js
.bowerrc
bower.json
*/

// Core variables and mixins
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/variables";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/mixins";

// Reset
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/normalize";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/print";

// Core CSS
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/scaffolding";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/type";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/code";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/grid";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/tables";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/forms";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/buttons";

// Components
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/component-animations";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/glyphicons";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/dropdowns";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/button-groups";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/input-groups";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/navs";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/navbar";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/breadcrumbs";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/pagination";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/pager";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/labels";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/badges";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/jumbotron";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/thumbnails";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/alerts";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/progress-bars";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/media";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/list-group";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/panels";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/wells";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/close";

// Components w/ JavaScript
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/modals";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/tooltip";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/popovers";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/carousel";

// Utility classes
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/utilities";
@import "../../vendors/bootstrap-sass-official/vendor/assets/stylesheets/bootstrap/responsive-utilities";

// Flatly theme from Bootswatch
@import "vendors/bootstrap";

app.scss

/*
Folder structure :

/frontend <-- AngularJS app
    /dist <-- Production ready files
        /assets
        /css
        /js
        index.html
    /src <-- Source files
        /app
        /assets
        /common
        /sass
            /vendors <-- Transformed css to scss partials
            _vendors.scss <-- import of all vendors
            app.scss <-- [CURRENT FILE]
        index.html
    /vendors <-- Bower default installation folder
gulpfile.js
.bowerrc
bower.json
*/

@import "vendors";

// Your styles go here !

gulpfile.js

/*
Folder structure :

/frontend <-- AngularJS app
    /dist <-- Production ready files
        /assets
        /css
        /js
        index.html
    /src <-- Source files
        /app
        /assets
        /common
        /sass
            /vendors <-- Transformed css to scss partials
            _vendors.scss <-- import of all vendors
            app.scss <-- Application styles
        index.html
    /vendors <-- Bower default installation folder
gulpfile.js <-- [CURRENT FILE]
.bowerrc
bower.json
*/

// Modules required
var gulp = require('gulp'),
    sass = require('gulp-ruby-sass'),
    minifycss = require('gulp-minify-css'),
    rename = require('gulp-rename');

// Copy and transform vendors' .css files to .scss partials
gulp.task('copy-css', function() {
    return  gulp.src(['vendors/bootswatch/flatly/bootstrap.css'/*, 'another/css/file.css'*/])
        .pipe(rename({prefix: "_", extname: ".scss"}))
        .pipe(gulp.dest('src/sass/vendors'));
});

// Compile and minify app.scss file to app.min.css
gulp.task('styles', ['copy-css'], function() {
    return gulp.src('src/sass/app.scss')
        .pipe(sass())
        .pipe(minifycss())
        .pipe(rename({suffix: '.min'}))
        .pipe(gulp.dest('dist/css'));
});

// Watch for .scss changes in src/sass folder
gulp.task('watch', function() {
    gulp.watch('src/sass/**/*.scss', ['styles']);
});