Hi,
The default heading font size is VW, it is responsive size. You could change it with px unit:
Please paste the custom CSS code to “Appearance > Customize: Additional CSS”:
.entry h1,.text_block h1,.ux-portfolio-template-intro h1,.slider-con-inn h1{ font-size:60px; }
.entry h2,.text_block h2,.ux-portfolio-template-intro h2,.slider-con-inn h2 { font-size:48px; }
.entry h3,.text_block h3,.ux-portfolio-template-intro h3,.slider-con-inn h3 { font-size:36px; }
.entry h4,.text_block h4,.ux-portfolio-template-intro h4,.slider-con-inn h4 { font-size:24px; }
Regards!