The CSS Cawlin linked to above works fine in Reading mode, but still no change for Source/Live Preview modes.
There are variables now that cover both Editing and Reading modes that make things a bit easier depending on the theme you are using. These are from Obsidian’s default CSS.
.theme-light, .theme-dark {
--link-unresolved-color: var(--color-yellow);
--link-unresolved-opacity: 0.7;
--link-unresolved-filter: none;
--link-unresolved-decoration-style: dotted;
--link-unresolved-decoration-color: var(--color-yellow);
}
The colors can written as var(--color-yellow);
yellow;
#ffc0cb;
etc.
The decoration-style can be dotted;
none;
solid;