Replace css.Sass with css.Build

[jmooring]

Use css.Build instead of css.Sass in layouts/_partials/func/style/GetMainCSS.html. This requires Hugo v0.158.0 or later.

The only reason the theme is using css.Sass is to create a source map.

By using css.Build we can eliminate the requirement to build with the extended edition, which is going to be discontinued at some point anyway.

[davidsneighbour]

Notes:

• needs min version in hugo.toml set to v0.158.0

Moving this into an issue.