Use `math.div` instead of `/` for division in SCSS

I got these warnings when running npm run build:

DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($WRAPPER-WIDTH-NARROW-PX, 2)

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

   ╷
54 │                 - $WRAPPER_WIDTH_NARROW_PX/2
   │                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    src/stylesheets/improved-corrections.scss 54:19  root stylesheet

👉 https://sass-lang.com/documentation/breaking-changes/slash-div

💡 git show --color-words=.

SimonAlling / better-sweclockers

Use `math.div` instead of `/` for division in SCSS #229