Поместите этот код в ваш CSS-файл, который находится после antd-CSS-файла:
.ant-menu-submenu > .ant-menu-submenu-title .ant-menu-submenu-arrow::before {
content: '';
position: absolute;
background: unset !important;
background-image: unset !important;
width: 0 !important;
height: 0 !important;
border-style: solid;
border-width: 6px 0 6px 6px;
border-color: #00d2be transparent transparent transparent;
transition: transform 0.3s ease-in-out !important;
transform: rotate(45deg) !important;
border-radius: 0 !important;
left: 0;
top: 50%;
}
.ant-menu-submenu-open > .ant-menu-submenu-title .ant-menu-submenu-arrow::before {
content: '';
position: absolute;
background: unset !important;
background-image: unset !important;
width: 0 !important;
height: 0 !important;
border-style: solid;
border-width: 6px 0 6px 6px;
border-color: #00d2be transparent transparent transparent;
transition: transform 0.3s ease-in-out !important;
transform: rotate(-45deg) !important;
border-radius: 0 !important;
left: 0;
top: 50%;
}
.ant-menu-submenu-inline > .ant-menu-submenu-title .ant-menu-submenu-arrow::after,
.ant-menu-submenu-vertical-left > .ant-menu-submenu-title .ant-menu-submenu-arrow::after,
.ant-menu-submenu-vertical-right > .ant-menu-submenu-title .ant-menu-submenu-arrow::after,
.ant-menu-submenu-vertical > .ant-menu-submenu-title .ant-menu-submenu-arrow::after {
content: none !important;
}