node_modules/gitbook-plugin-prism/prism-ebook.css

/*
 * Prism themes contain a ::selection selector that was being applied to <code>
 * blocks in PDFs.
 *
 * @See https://github.com/gaearon/gitbook-plugin-prism/issues/11
*/
pre[class*="language-"]::selection, pre[class*="language-"] ::selection,
code[class*="language-"]::selection, code[class*="language-"] ::selection {
  background: none;
}

/*
 * @See https://github.com/gaearon/gitbook-plugin-prism/issues/11#issuecomment-262058733
*/
.page .section pre code {
    background-color: none;
}