set variables for box-shadow

packages/core/scss/bootstrap/_variables.scss

@@ -25,12 +25,29 @@ $warning-border-subtle:       mix(#fff, $warning, 70%) !default;
 $danger-border-subtle:        mix(#fff, $danger, 70%) !default;
 
 
+// Options
+//
+// Quickly modify global styling by enabling or disabling optional features.
+
+$enable-shadows: true;
+$box-shadow-inset: inset 0 0 rgba(black, 0); // set invisible value for inset
+
+
+// Buttons
+//
+// For each of Bootstrap's buttons, define text, background, and border color.
+
+$btn-box-shadow: 0;
+$btn-active-box-shadow: 0;
+
+
 // Links
 //
 // Style anchor elements.
 
 $link-decoration: none;
 
+
 //== Typography
 //
 //## Font, line-height, and color for body text, headings, and more.