ps in the meantime i’ve checked the css deeply and found this for the last menu item, which works great, too.
.x-navbar-fixed-top-active .masthead-inline .desktop .x-nav>li:last-child>a
but when using this for the mobile version. the button is far to long (as long as the menu). how to fix this?

.x-navbar-fixed-top-active .masthead-inline .mobile .x-nav>li:last-child>a {
margin-top: 29px;
margin-left: 12px;
margin-right: 20px;
background: #447b19;
padding: 9px 20px 0px 15px;
border-radius: 20px;
}
i have tried to use margin-right for this. but as it is dynamic it does not work.
dev: http://gelenkalbin.oblaum.com
thanks
oli