fix(css): add workaround for postcss value parser error (#1524)

postcss-values-parser returns a syntax error when a negative value is
provided in a `calc` function after a CSS variable.

This is not an issue with Boosted itself, but a workaround to allow
projects using postcss-values-parser to keep upgrading and compiling
Boosted

Ref: shellscape/postcss-values-parser#138

scss/_navbar.scss

@@ -293,7 +293,7 @@
 // Button for toggling the navbar when in its collapsed state
 .navbar-toggler {
   padding: var(--#{$prefix}navbar-toggler-padding-y) calc(var(--#{$prefix}navbar-toggler-padding-x) / 2) var(--#{$prefix}navbar-toggler-padding-y) var(--#{$prefix}navbar-toggler-padding-x); /* stylelint-disable-line function-disallowed-list */ // Boosted mod
-  margin-right: calc(var(--#{$prefix}navbar-toggler-padding-x) / -2); /* stylelint-disable-line function-disallowed-list */ // Boosted mod
+  margin-right: calc(-.5 * var(--#{$prefix}navbar-toggler-padding-x)); /* stylelint-disable-line function-disallowed-list */ // Boosted mod
   @include font-size(var(--#{$prefix}navbar-toggler-font-size));
   line-height: 1;
   color: var(--#{$prefix}navbar-color);