Menu button gradient?

you can see it here even with animation on Main Menu “Impressum” : https://webers-testseite.de/
If you do not need that animation – remove that part from rules and set the background-size to 100%.

You can select that menu-item via ID
and do something like this:

#menu-item-ID .avia-menu-text {
  color: #fff;   /**** perhaps an !important is necessary ****/
  background: linear-gradient(-45deg,#FFA63D,#FF3D77,#338AFF,#3CF0C5);
  background-size: 600%;
  border: none !important;
  animation: anime 14s linear infinite;
  padding: 11px 20px !important;
  border-radius: 0 0 10px 10px !important;
}

@keyframes anime {
  0% { background-position:0% 50% }
  50% { background-position:100% 50% }
  100% { background-position:0% 50% }
}