box-shadow @mixin is conflicting with Bootstrap's box-shadow @mixin · Issue #2622 · snapappointments/bootstrap-select · GitHub
Stripe-style box shadow mixin
Bootstrap on Twitter: "We'll be bringing CSS variables to more components in each minor v5 release to make customization easier and even reduce file size in some cases. Consider this before and
Box shadow scss mixin
box-shadow inset problem · Issue #13 · matthieua/sass-css3-mixins · GitHub
Getting Deep Into Shadows | CSS-Tricks - CSS-Tricks