You need to install and activate the child theme in order to do this changes. Please have a look on this theme guide below:
https://showthemes.zendesk.com/docs/khore/how-to-activate-a-child-theme/
Login to the admin dashboard, Navigate Appearance -> Editor.
On the stylesheet, please add the following style.
.menu__item.menu__item_opened a {
color: #fff;
}
Update the file