SCSS parse warnings

After the last update to the gulpfile, there are SCSS parse warnings.

Example:

```
Deprecation Warning [global-builtin]: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use color.mix instead.

More info and automated migrator: https://sass-lang.com/d/import

╷
207 │ @return mix(white, $color, $weight);
```

Parsing works, but would be good to fix anyway.
(bu for now I already spent way too much time on trying to get the blog social share icons replaced...)