Using Skin on Website with TinyMCE Cloud (v6) - tinymce

Question

I'm using Tiny Cloud on my website to bring up a TinyMCE editor. I want to skin it, because the dark mode looks ugly on my dark website. I've been able to initiate it properly, and get the standard oxide-dark skin to work.
<script>
tinymce.init({
selector: '#mytextarea',
plugins: 'lists link image autolink autosave autoresize save searchreplace wordcount',
toolbar: 'undo redo copy cut paste | blocks | bold italic underline forecolor | alignleft aligncenter alignright | removeformat',
menubar: '',
skin: 'oxide-dark',
content_css: 'dark'
});
</script>
... and later...
<textarea id="mytextarea">Hello, World!</textarea>
I then used the skin editor at http://skin.tiny.cloud/t5/ (which I know is v5, but that's what v6 pointed to). I downloaded the result which gave me a ZIP file. I saved the "skins" folder that was in that ZIP file into the root HTML folder of my website. I then update the initiation of tinymce as follows;
<script>
tinymce.init({
selector: '#mytextarea',
plugins: 'lists link image autolink autosave autoresize save searchreplace wordcount',
toolbar: 'undo redo copy cut paste | blocks | bold italic underline forecolor | alignleft aligncenter alignright | removeformat',
menubar: '',
skin: 'Opus',
content_css: 'skins/content/Opus/content.css',
});
</script>
On my site, it doesn't bring up any textbox at all (usually when I screwed something up it would at least bring up an old-school html textbox, but this isn't even showing that). It looks like it's supposed to, and even gives it space on the page, but nothing is visible. What step did I miss in getting this to work?