@SamAdams from the Typomagical theme:
a,
.cm-hmd-internal-link,
.cm-link,
.markdown-preview-view .internal-link {
text-decoration: none;
font-variant: small-caps;
}
That covers both internal and external links. Note the small caps transformation for links which I find a nice change, but if you don’t like it just delete that line.
Last but not least, at the risk of seeming immodest, over the past 1½ years I have collected a lot of CSS snippets that someone put in a Github repository - you might find quite a few useful ones. Some of them won’t work in Live Preview, other will.
Click on the Github link in the uppermost comment on this forum page, and it’ll take you there.
Enjoy !