Index: wp-content/themes/twentynineteen/style-editor.css
===================================================================
--- wp-content/themes/twentynineteen/style-editor.css	(revision 58682)
+++ wp-content/themes/twentynineteen/style-editor.css	(working copy)
@@ -1633,3 +1633,6 @@
 .wp-block-post-author__avatar img {
   border-radius: 100%;
 }
+
+/** === Details Block === */
+details[style*="font-size"] p {font-size: inherit;}
\ No newline at end of file
Index: wp-content/themes/twentynineteen/style-editor.scss
===================================================================
--- wp-content/themes/twentynineteen/style-editor.scss	(revision 58682)
+++ wp-content/themes/twentynineteen/style-editor.scss	(working copy)
@@ -1079,3 +1079,13 @@
 .wp-block-post-author__avatar img {
 	border-radius: 100%;
 }
+
+/** === Details Block === */
+details {
+	&[style*="font-size"] {
+	  p {
+		font-size: inherit;
+	  }
+	}
+  }
+  
\ No newline at end of file