I got it to work by using the following:
'plugins': [
"themes"
],
'core': {
"themes": {
"name": 'default-dark',
"dots" : true,
"responsive" : true,
"stripes": false
},
....
Be sure that the "themes" section is under "core" not a sibling of "core" and that you have the "themes" plugin in the "plugins" section.
Also on the page that loads your css files, be sure to have the correct path to the default-dark/style.min.css.
--
Dez.