Breaks pdf, epub, mobi generation

[looeee]

My book.json:

{
  "author": "My name",
  "title": "My title",
  "description": "",
  "plugins": [
    "katex",
    "theme-gestalt",
    "-theme-default",
    "styles-sass-fix"
  ],
  "pluginsConfig": {
    "theme-gestalt": {
      "logo": "/assets/logo_1024.png",
      "favicon": "/assets/favicon/favicon-96x96.png",
      "baseUrl": null,
      "excludeDefaultStyles": true,
      "doNotHideChildrenChapters" : false
    }
  },
  "styles": {
    "website": "./styles/base.scss"
  }
}

./styles/base.scss looks like:

@import "../node_modules/gitbook-plugin-theme-gestalt/src/scss/variables";

@import "../node_modules/gitbook-plugin-theme-gestalt/src/scss/all";

When I run any of the pdf, epub or mobi commands, such as

gitbook pdf ./ ./mybook.pdf

I get:

nfo: 10 plugins are installed
info: 8 explicitly listed
info: loading plugin "katex"... OK
info: loading plugin "styles-sass-fix"... OK
info: loading plugin "highlight"... OK
info: loading plugin "search"... OK
info: loading plugin "lunr"... OK
info: loading plugin "sharing"... OK
info: loading plugin "fontsettings"... OK
info: loading plugin "theme-gestalt"... OK
info: found 3 pages
info: found 42 asset files
info: compile sass file:  ./styles/base.scss
error: error while generating page "README.md":

TemplateError: Error compiling template "ebook/page.html": Not found

If I reset my book.json back to

{
  "author": "Lewy Blue",
  "title": "Web Animation with three.js",
  "description": "",
  "plugins": [
    "katex"
  ]
}

Then everything works again.