What does the SassError message "Invalid CSS after @include mat: expected 1 selector or at-rule, was .core();" in relation to Angular material mean?

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The SassError message indicates that there is an error in the CSS code related to using the @include mat syntax with Angular Material. Specifically, it expected to find 1 selector or at-rule after the @include mat syntax, but instead found a reference to .core(). This suggests that there may be an issue with the implementation of the Angular Material theme in the code.