The current version of Carbon Components Svelte ( https://github.com/carbon-design-system/carbon-components-svelte ) doesn't yet support theming the header, sidebar etc. This is a simple Sveltekit app boilerplate that uses PostCSS and allows for global CSS overrides in the src/styles/app.css file.
git clone https://github.com/panagapko/svelte-carbon-boilerplate.git svelte-carbon-boilerplate
cd svelte-carbon-boilerplate
npm install
npm run dev