Following the installation guide for Rollup bundler, after installing the required npm packages, when postcss() is added to rollup.config.js the following error occurs.

[mgruen]

[!] (plugin svelte) ParseError: Identifier is expected
node_modules\svelma\src\components\Field.svelte
95: <style lang="scss">
96: .field {
97: &.is-grouped {
^
98: .field {
99: flex-shrink: 0;
ParseError: Identifier is expected
at error (C:\dev\svelte1\node_modules\svelte\src\compiler\utils\error.ts:25:16)
at Parser$1.error (C:\dev\svelte1\node_modules\svelte\src\compiler\parse\index.ts:100:3)
at Object.read_style [as read] (C:\dev\svelte1\node_modules\svelte\src\compiler\parse\read\style.ts:21:11)
at tag (C:\dev\svelte1\node_modules\svelte\src\compiler\parse\state\tag.ts:203:27)
at new Parser$1 (C:\dev\svelte1\node_modules\svelte\src\compiler\parse\index.ts:52:12)
at parse (C:\dev\svelte1\node_modules\svelte\src\compiler\parse\index.ts:216:17)
at compile (C:\dev\svelte1\node_modules\svelte\src\compiler\compile\index.ts:79:14)
at Object.transform (C:\dev\svelte1\node_modules\rollup-plugin-svelte\index.js:105:21)
at C:\dev\svelte1\node_modules\rollup\dist\shared\rollup.js:18765:25

[handsome-b-wonderful]

I needed to make a few updates to get this working.

install postcss ^8.2.4

npm install postcss --save-dev

update your rollup.config.js

import postcss from 'rollup-plugin-postcss'
import preprocess from 'svelte-preprocess'
....

	plugins: [
		...
		svelte({
			...
			preprocess: preprocess(),
		}),

		postcss(),
               ...