This is what I found worked when I was fixing my Neospring theme's CSS after the new update

For page dolls and image borders remove the /api/v0/util/ext/image?img= in url("/api/v0/util/ext/image?img=https://image.link.com"), it should just be url() with your image link in the brackets, url("https://image.link.com") (The recommended way to use url() in CSS is to add quotes around your image link but Neospring allows it without quotes)

For fonts you remove the /api/v0/util/ext/image?img= as well, if it still doesn't work you can try adding !important before the semi-colon under the CSS that sets your font

@font-face {
        font-family: font;
        src: url("https://font.link.com");
}

* {
        font-family: font !important;
}

Do keep in mind that /api/v0/util/ext/image?img= will be added after saving the theme and refreshing, so each time you want to update your theme you'll have to remove those again otherwise it will stack and break your code

Edit: The above has been fixed https://neospring.org/+r/5cfbc1066b