Great, that did help a lot. I also added:
.main-navigation ul ul a:hover,
.main-navigation ul ul a:focus {
color: #ffdd00 !important;
}
to fix the hover. It didn’t actually become white though, probably because of the RGBA specification earlier in the CSS. As you can see on https://sportplatformwaddinxveen.nl/sportservices/ it is now a shade of gray (60% alpha as specified). I’ve tried getting it to really white but couldn’t make that work. However the menu is perfectly readable now on both desktop and mobile.
Thanks for the assistance!