Thank you so much for your help!
Here is the full code in Custom CSS:
@font-face {
font-family: 'Mango';
src: url('https://static1.squarespace.com/static/599ba7cf15d5db327ad86d5d/t/6164796cc1a31d18d9c0cdcb/1633974636221/Mango.ttf'), url(https://static1.squarespace.com/static/599ba7cf15d5db327ad86d5d/t/61647987c6d0580035320eeb/1633974663996/Mango.woff;),
url('https://static1.squarespace.com/static/599ba7cf15d5db327ad86d5d/t/61647db7e1770d726d80791d/1633975735619/Mango.woff2';)
}
.header-title-text {
font-family: 'Mango' !important;
}
.header-nav-item {
font-family: 'Mango';
}
h1 {font-family: 'Mango';}
Header-branding-title {
font-family: 'Mango';}
Header-nav-title {font-family: 'Mango';} Header-nav-secondary {font-family: 'Mango';}