Apply box-sizing: border-box to all Amplify theme children (#615)

* Apply box-sizing: border box to all Amplify theme children

Author: reesscot

Diff for: .changeset/shaggy-ladybugs-crash.md

@@ -0,0 +1,5 @@
+---
+"@aws-amplify/ui": patch
+---
+
+Apply `box-sizing: border-box` styling to all Amplify theme children

Diff for: packages/ui/src/theme/css/styles.scss

@@ -17,6 +17,12 @@ html {
   all: unset; /* protect against external styles */
 }
 
+// Must be defined AFTER `[class*='amplify']
+// or rules will be unset to initial/default
+[data-amplify-theme] * {
+  box-sizing: border-box;
+}
+
 .sr-only {
   position: absolute !important;
   width: 1px !important;