Hi

Thanks for contacting us. Use this custom CSS

.sp-megamenu-parent > li > a{
  font-size:18px !important;
  font-weight:bold !important;
}

Go to system--> https://prnt.sc/tDbSUJh6LiGu --> Default template-->Template options--> https://prnt.sc/A5nlsBvCQsqs And add your CSS there.

-Regards.