fix: errors when defining typography with css variables

* Fixes invalid operation error for font-size calculation with css variables

Author: Peter Balas

src/material/core/typography/_typography.scss

@@ -96,7 +96,7 @@
   // and h6 at 0.67em.
   .mat-h5, #{$selector} h5 {
     @include mat-typography-font-shorthand(
-      mat-font-size($config, body-1) * 0.83,
+      calc(mat-font-size($config, body-1) * 0.83),
       mat-font-weight($config, body-1),
       mat-line-height($config, body-1),
       mat-font-family($config, body-1)
@@ -107,7 +107,7 @@
 
   .mat-h6, #{$selector} h6 {
     @include mat-typography-font-shorthand(
-      mat-font-size($config, body-1) * 0.67,
+      calc(mat-font-size($config, body-1) * 0.67),
       mat-font-weight($config, body-1),
       mat-line-height($config, body-1),
       mat-font-family($config, body-1)