Changing the button color on the mobile navigation menu

Site URL: https://www.decorationvolckaert.be

Hello everyone, I've got an issue with the color of my button on the mobile navigation menu. In fact, it appears blue on the desktop version but black on the mobile menu which makes it almost unreadable. Can somebody have a solution for this problem ? I can't find where I can change this color just for the mobile version. Thank you 

Sorry if my English is not perfect, I'm french native 😉

Add to Home > Design > Custom CSS

.header-menu-cta a {
    border-color: #3bd9f1 !important;
    color: #3bd9f1 !important;
}