Unfortunately, optional parameters need to be optional in both .<filename>_vars() and .<filename>_main()

customize.dist/src/less2/include/checkmark.less

@@ -11,7 +11,7 @@
     @checkmark-radio-size: @checkmark-dim1 * 3;
 }
 
-.checkmark_main(@size) {
+.checkmark_main(@size: 20px) {
     --LessLoader_require: LessLoader_currentFile();
     .checkmark_vars(@size);
     --checkmark-size: @checkmark-size;