Every YJSG based template comes with custom.css. Custom.css is loaded as a very last file in header thus you have an option to override any CSS declaration from your template or extensions. We strongly encourage you to use this file to make any modification you might need.
This way when there is a template update your overrides will still be in effect.

Once you activate the custom.css in template manager, custom.css file will be placed in site_root/templates/yjsg_template_name/css folder.

change module and article title font size on screens smaller than 980px:

@media screen and (max-width: 979px){
.module_title,
.article_title{
font-size:50px;
}
}