How to exclude specific css file from aggregation
Similar to Drupal 7, the setting preprocess key to FALSE will prevent a JS or CSS library from being aggregated.

mylibraryname:
  css:
    theme:
      css/custom-style.css: {preprocess: false}


Same works for JavaScript

cuddly-slider:
  version: 1.x
  js:
    js/cuddly-slider.js: {preprocess: false}
  dependencies:
    - core/jquery
    - core/drupalSettings


As documented on Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 module