Hi,

1/ You can select this font to apply for all your heading titles by default via Customize > General Options > "Font for Heading Titles"

2/ For apply custom css for single post title, you can use the Custom CSS code like this:

.container-single .single-post-title{
    font-size: 24px;
    font-weight: bold;
    font-style: normal;
    letter-spacing: -1.9px;
}

Best Regards,

PenciDesign