[css] SASS - use variables across multiple files

Create an index.scss and there you can import all file structure you have. I will paste you my index from an enterprise project, maybe it will help other how to structure files in css:

@import 'base/_reset';

@import 'helpers/_variables';
@import 'helpers/_mixins';
@import 'helpers/_functions';
@import 'helpers/_helpers';
@import 'helpers/_placeholders';

@import 'base/_typography';

@import 'pages/_versions';
@import 'pages/_recording';
@import 'pages/_lists';
@import 'pages/_global';

@import 'forms/_buttons';
@import 'forms/_inputs';
@import 'forms/_validators';
@import 'forms/_fieldsets';

@import 'sections/_header';
@import 'sections/_navigation';
@import 'sections/_sidebar-a';
@import 'sections/_sidebar-b';
@import 'sections/_footer';

@import 'vendors/_ui-grid';

@import 'components/_modals';
@import 'components/_tooltip';
@import 'components/_tables';
@import 'components/_datepickers';

And you can watch them with gulp/grunt/webpack etc, like:

gulpfile.js

// SASS Task

var gulp = require('gulp');
var sass = require('gulp-sass');
//var concat = require('gulp-concat');
var uglifycss = require('gulp-uglifycss');
var sourcemaps = require('gulp-sourcemaps');

gulp.task('styles', function(){
    return gulp
            .src('sass/**/*.scss')
            .pipe(sourcemaps.init())
            .pipe(sass().on('error', sass.logError))
            .pipe(concat('styles.css'))
            .pipe(uglifycss({
                "maxLineLen": 80,
                "uglyComments": true
            }))
            .pipe(sourcemaps.write('.'))
            .pipe(gulp.dest('./build/css/'));
});

gulp.task('watch', function () {
    gulp.watch('sass/**/*.scss', ['styles']);
});

gulp.task('default', ['watch']);