Correct the syntax in hero is-bold colour variants

[bagedevimo]

While the prior syntax does produce valid css, it's not quite valid scss and so produces warnings in various pre and post-processing pipelines which various upstream consumers of Bulma may use.

This is a bugfix.

Proposed solution

By using the correct scss syntax, we just prevent downstream warnings and errors, and make this file consistent with the rest of the bulma codebase., for example:

#30 114.5 WARNING in css/partner_portal-d7c813d1.css
#30 114.5 css/partner_portal-d7c813d1.css from Css Minimizer plugin
#30 114.5 postcss-calc:: Lexical error on line 1: Unrecognized text.
#30 114.5 
#30 114.5   Erroneous area:
#30 114.5 1: var(--bulma-hero-h) - $hero-gradient-h-offset
#30 114.5 ^........................^ webpack://./node_modules/bulma/sass/layout/hero.scss:146:12

Testing Done

Generated css output has no difference before and after this change.

Changelog updated?

No.