Add box-sizing: content-box for compatibility (#1229)

Some CSS resets (including Bootstrap's) contain *::after{ box-sizing: border-box; }

This breaks the checkbox component style. Therefor add box-sizing: content-box explicitly to increase compatibility.

packages/theme-default/src/checkbox.css

@@ -36,6 +36,7 @@
       }
 
       &::after {
+        box-sizing: content-box;
         content: "";
         border: 2px solid var(--checkbox-checked-icon-color);
         border-left: 0;