Update alt background in paper theme

Done

Update alt background in paper theme from white to #EBEBEB (to be darker than default)

Fixes #5197

Open demo
Make sure that the highlighted colour on paper theme is correct
Check other affected components:
- https://vanilla-framework-5198.demos.haus/docs/examples/patterns/strips/highlighted

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

[x] PR should have one of the following labels to automatically categorise it in release notes:
- Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
[x] Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
- if CSS class names are not changed it can be bugfix relesase (x.x.X)
- if CSS class names are changed/added/removed it should be minor version (x.X.0)
- see the wiki for more details
[x] Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.