Can't change font size of the main menu sub-level titles

Dear Enfold team,

We are having problems in trying to change the font size of the main menu sub-level titles so they will appear smaller than the main menu title.
We have played around with the advance styling to make the ‘main menu links’ font size to 21px and ‘main menu sub-level links’ font size to 15px,
but nothing happens. ( we have cleared cache).

We copied another enfold working website child theme to work from. Here is the relevant current custom CSS one can see related to the main menu color/ font etc:

.header_color .main_menu ul > li:hover > a .avia-menu-text,
.header_color .main_menu ul > li > a:hover .avia-menu-text {
color: #C55A11 !important;
}

/* Main menu subtext */

.header_color .main_menu ul > li > a .avia-menu-text,
.header_color .main_menu ul > li > a .avia-menu-subtext {
font-size: 18px !important;
}

Any advice would be most welcome.

Thanks