You need to be a bit more specific with the CSS. Give this a try, for example:
/* source mode and live preview font */
.markdown-source-view.mod-cm6 .cm-scroller {
font-family: 'Macondo', serif;
color: cyan;
}
/* rendered & reading view font */
.markdown-preview-view, .markdown-rendered {
font-family: 'Iosevka-Comfy-Duo';
}