As far as I can tell, Obsidian simply cannot differentiate between unresolved and resolved links. However, I have implemented a fix for underlines in Live Preview mode which fixes internal links having underlines.

Hi there, you can distinguish unresolved links. This is what I use to make them wavy-underlined and a complementary colour.

Got this from the forum I think, but adapted: https://forum.obsidian.md/t/how-can-i-make-unresolved-link-display-differently/45171/2

As opposed to that answer in #133 (which I'm not familiar with), my snippet below only works in Edit Mode. Not in Reading Mode. But there must be a class for both, I assume. (Also sorry if I misunderstood, and this wasn't what either of you were talking about!)

.markdown-source-view.mod-cm6 .is-unresolved {
    color: orange;
    font-style: italic;
    text-decoration-style: wavy;
    text-decoration-line: underline;
}