Hello,
For categories https://prnt.sc/ql8dvt you can add this code in Theme Options > Theme Custom CSS > Custom CSS for mobile:
.category-grid .categories-mask h4 {
font-size: 10px !important;
}
http://prntscr.com/ql8m0b
For headings, like https://prnt.sc/ql8dmi , you can add custom class http://prntscr.com/ql8nds and the next code in Cusrom CSS for moblie:
.mobile-heading {
font-size: 20px;
}
Regards