The CSS will, of course, vary depending on a few things.
Are you using a community theme? If so, which one?
Do you want to reduce the spacing in Live Preview, Reading view, or both?
You could give this a try to start. You should see a change in Live Preview and Reading views.
.markdown-rendered ul.dataview.list-view-ul {
margin-block-start: 0.5em; /* default is 1em */
}