Reply To: design the home page

Hello,

Navigate to Products > Edit Product and change the heading Style.

See Screenshots for better understanding:
https://ibb.co/r56bpd4

For the Related product title, Try to add the following custom css in Theme Settings > Custom CSS > Global Custom CSS:

.wd-el-title {
    font-size: 40px !important;
}

Also your product title in H1 in a single product page.
https://ibb.co/SwzYptf

Best Regards.