source-mode-editor-font.css

/* @settings

name: Source Mode Editor Font
id: source-mode-editor-font
settings:
    -
        id: source-mode-editor-font
        title: Source Mode Editor Font
        description: Font to use exclusively for the source mode editor view
        type: variable-text
        default: 'var(--font-monospace)'
*/

/* Not sure if .mod-cm6 is needed */

.view-content > .markdown-source-view:not(.is-live-preview),
.view-content
  > .markdown-source-view.mod-cm6:not(.is-live-preview)
  .cm-scroller {
  font-family: var(--source-mode-editor-font, var(--font-monospace));
}