I see you have a minification and concatenation plugin running so it’s hard for me to see which selector and styles you have set on the menu via Microthemer. Would you mind sending access for the affected site via our secure contact form?
Just from looking at the frontend, I can’t see any obvious reason why it would be difficult to override the theme’s CSS:
.et_pb_menu_0_tb_header.et_pb_menu ul li a {
font-size: 0.9vw;
}