add required missing defaults

Author: cscheid

src/resources/formats/html/_quarto-rules.scss

@@ -700,6 +700,7 @@ $body-bg: #fff !default;
 $primary: #468 !default;
 $secondary: #999 !default;
 $font-family-sans-serif: sans-serif !default;
+$font-weight-base: 400 !default;
 
 /* SCSS variables