Index: src/wp-content/themes/twentytwenty/assets/css/editor-style-block-rtl.css
===================================================================
--- src/wp-content/themes/twentytwenty/assets/css/editor-style-block-rtl.css	(revision 58095)
+++ src/wp-content/themes/twentytwenty/assets/css/editor-style-block-rtl.css	(working copy)
@@ -134,6 +134,10 @@
 
 /* Colors ------------------------------------ */
 
+:where(.editor-styles-wrapper .has-background) {
+	box-sizing: border-box;
+}
+
 .editor-styles-wrapper .has-text-color a {
 	color: inherit;
 }
Index: src/wp-content/themes/twentytwenty/assets/css/editor-style-block.css
===================================================================
--- src/wp-content/themes/twentytwenty/assets/css/editor-style-block.css	(revision 58095)
+++ src/wp-content/themes/twentytwenty/assets/css/editor-style-block.css	(working copy)
@@ -134,6 +134,10 @@
 
 /* Colors ------------------------------------ */
 
+:where(.editor-styles-wrapper .has-background) {
+	box-sizing: border-box;
+}
+
 .editor-styles-wrapper .has-text-color a {
 	color: inherit;
 }