Unknown at rule @mixincss(unknownAtRules)" when using Mantine Buttons

## Issue: "Unknown at rule @mixincss(unknownAtRules)" when using Mantine Buttons

### Description
When following the Mantine documentation for [Buttons](https://ui.mantine.dev/category/buttons/), I encountered the following error:

This issue occurs when trying to use the recommended CSS setup for styling buttons and other components.

### Steps to reproduce
1. Follow the button example in the Mantine documentation: [https://ui.mantine.dev/category/buttons/](https://ui.mantine.dev/category/buttons/).
2. Encounter the error `Unknown at rule @mixincss(unknownAtRules)` when trying to compile the CSS.

### Expected behavior
The styling for Mantine components (such as buttons) should work without errors, as per the documentation.

### Actual behavior
The error appears that's it

### Environment
- Mantine version: latest
- vike framework using bâti
- Browser: CHROME

Start with the Mantine Buttons documentation linked in the issue and reproduce the reported CSS setup in the stated Vike/bâti environment. Trace where the `@mixincss` at-rule is introduced and verify that the button example compiles without the unknown-at-rule error.

Written by the indexing model from the issue text.