Theme's css and js files are not being loaded
englishtrainer.info.yml:

name: Englishtrainer
type: theme
core: 8.x
version: '1.0.0'
libraries:
  - englishtrainer/base
regions:
  header: Header


No need for package

englishtrainer.libraries.yml:

base:
  version: VERSION
  css:
    theme:
      css/css-test.css: {}
  js:
    js/js-test.js: {}


Use base to start things off and the VERSION keyword will take the version info from englishtrainer.info.yml.