Prerequisites

# 1. install dependencies
npm i


# 2. build app
npm run build -w packages/svelte-kit-app

Reproduce

In packages/svelte-kit-app we use the packages/svelte-kit-library in src/routes/+page.svelte. First use the module import in line 7.

import { Heading } from "svelte-kit-library";

# build app
npm run build -w packages/svelte-kit-app

Now check the output packages/svelte-kit-app/.svelte-kit. If you search for .unwanted which is styling of the Unwanted component of packages/svelte-kit-library you realize that styling exists in the build result. However, we have not imported the component at any point in the project.
Now change the import statement and use explicit import in line 10.

import Heading from "svelte-kit-library/Heading.svelte";

# build app
npm run build -w packages/svelte-kit-app

Now the .unwanted class is no longer included in the build. It seems that the tree shaking does not work correctly at this point.

Rebuild library

# 1. build library
npm run package -w packages/svelte-kit-library

# 2. pack library
npm pack -w packages/svelte-kit-library

# 3. install library
npm i svelte-kit-library-0.0.1.tgz -w packages/svelte-kit-app