Describe the bug
astro-purgeCSS does not remove unused SCSS classes

To Reproduce

1. Create a .astro file
2. Create a custom.scss file
3. Import other .scss files in it
4. Import our custom.scss file in index.astro
5. Run npx astro build
6. No processing is done and this log is given -

11:23:06 [astro-purgecss] 📦 Running in 'static' mode
No files found from the passed PurgeCSS option 'content'.
11:23:06 [astro-purgecss] ℹ️  No CSS files found to process

Expected behavior
I expected unused classes to be removed

Additional context
I am using Bootstrap. I customise the components using SCSS in the custom.scss file, I then import this file in index.astro. I run npx run build but unused classes don't get removed