userchrome.css

/* Render horizontal lines (made with \-\-\- or \*\*\*) as an actual line across the editor. */
div.CodeMirror .cm-hr {
  border-top: 1px solid #777;
  display: block;
  line-height: 0px;
}

/* If using the Enforce CSS option, then use the below CSS */
div.CodeMirror pre.cm-rm-hr.CodeMirror-line  {
  border-top: 1px solid #777;
	line-height: 0px;
}

/* Reduce the size and visibility of the header hash tags. */
/* The additional CSS option must be enabled */
div.CodeMirror .cm-header.cm-rm-header-token {
	font-size: 0.9em;
	color: grey;
}

/* Additionally, this CSS unindent the "#" characters to align */
/* the header text with the rest of the text */
div.CodeMirror .cm-header.cm-rm-header-token {
	color: #cccccc;
	font-size: 0.9em;
	margin-left: -50px;
	max-width: 50px;
	width: 50px;
	overflow: hidden;
	display: inline-block;
	text-align: right;
	opacity: 0.5;
}

/* Reduces the intensity of the italics (emph) and bold (strong) markdown tokens */
div.CodeMirror .cm-em.cm-rm-em-token,
div.CodeMirror .cm-strong.cm-rm-strong-token {
	opacity: 0.5;
}

/* This is also available for highlight and strikethrough, but it doesn't look very good */
/*
.cm-rm-highlight.cm-rm-highlight-token,
.cm-strikethrough.cm-rm-strike-token,
*/

/* Add a vertical bar to the left side of quote blocks so they match the viewer */
pre.cm-rm-blockquote.CodeMirror-line {
  border-left: 4px solid var(--joplin-code-border-color);
  opacity: 0.7;
}

pre.cm-rm-blockquote span.cm-quote + span.cm-quote {
  opacity: 1;
}

/* WARNING: the below code will hide the > from quotes, use at your own risk */
/*
pre.cm-rm-blockquote span.cm-quote.cm-rm-list-token {
  opacity: 0;
}
*/

/* Changes the monospace font used for tabes/checkboxes */
/* All fonts option must be enabled */
div.CodeMirror .cm-overlay.cm-rm-monospace {
	font-family: monospace !important;
}

/* strikeout and dim the text of a checked checkbox */
div.CodeMirror span.cm-rm-checkbox.cm-property + span.cm-rm-checkbox ~ span.cm-rm-checkbox {
	text-decoration: line-through;
	opacity: 0.7;
}
/* Uncomment the below sections to include the checkbox itself  */
/*
span.cm-rm-checkbox.cm-property + span.cm-rm-checkbox {
	text-decoration: line-through;
	opacity: 0.7;
}
span.cm-rm-checkbox.cm-property {
	text-decoration: line-through;
	opacity: 0.7;
}
*/

/* Requires the "highlight background of current line option to be enabled */
div.CodeMirror .CodeMirror-activeline .CodeMirror-activeline-background.CodeMirror-linebackground {
  background: lightgrey !important;
}