No background for highlighted code

[constgen]

I use Webpack 3, so have to use mdx-loader@2.2.1

I followed the instructions about importing a theme https://github.com/frontarm/mdx-util/tree/master/packages/mdx-loader#syntax-highlighting . But the result is that background styles are not applied because the generated markup doesn't preserve theme selectors https://github.com/PrismJS/prism/blob/master/themes/prism-tomorrow.css#L39

:not(pre) > code[class*="language-"],
pre[class*="language-"] {
	background: #2d2d2d;
}

Built result