CSS derived from sindresorhus/github-markdown-css to suit MacDown. Instructions (you can skip steps 1 to 3!): https://twitter.com/HeathRaftery/status/1517372992757518336

github-markdown-light.css

body {
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
  margin: 0;
  color: #24292f;
  background-color: #ffffff;
  font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji";
  font-size: 16px;
  line-height: 1.5;
  word-wrap: break-word;
  padding-top: 10px;
  padding-bottom: 10px;
  padding: 30px;
}

.octicon {
  display: inline-block;
  fill: currentColor;
  vertical-align: text-bottom;
}

h1:hover .anchor .octicon-link:before,
h2:hover .anchor .octicon-link:before,
h3:hover .anchor .octicon-link:before,
h4:hover .anchor .octicon-link:before,
h5:hover .anchor .octicon-link:before,
h6:hover .anchor .octicon-link:before {
  width: 16px;
  height: 16px;
  content: ' ';
  display: inline-block;
  background-color: currentColor;
  -webkit-mask-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' version='1.1' aria-hidden='true'><path fill-rule='evenodd' d='M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z'></path></svg>");
  mask-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' version='1.1' aria-hidden='true'><path fill-rule='evenodd' d='M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z'></path></svg>");
}

details,
figcaption,
figure {
  display: block;
}

summary {
  display: list-item;
}

[hidden] {
  display: none !important;
}

a {
  background-color: transparent;
  color: #0969da;
  text-decoration: none;
}

a:active,
a:hover {
  outline-width: 0;
}

abbr[title] {
  border-bottom: none;
  text-decoration: underline dotted;
}

b,
strong {
  font-weight: 600;
}

dfn {
  font-style: italic;
}

h1 {
  margin: .67em 0;
  font-weight: 600;
  padding-bottom: .3em;
  font-size: 2em;
  border-bottom: 1px solid hsla(210,18%,87%,1);
}

mark {
  background-color: #fff8c5;
  color: #24292f;
}

small {
  font-size: 90%;
}

sub,
sup {
  font-size: 75%;
  line-height: 0;
  position: relative;
  vertical-align: baseline;
}

sub {
  bottom: -0.25em;
}

sup {
  top: -0.5em;
}

img {
  border-style: none;
  max-width: 100%;
  box-sizing: content-box;
  background-color: #ffffff;
}

code,
kbd,
pre,
samp {
  font-family: monospace,monospace;
  font-size: 1em;
}

figure {
  margin: 1em 40px;
}

hr {
  box-sizing: content-box;
  overflow: hidden;
  background: transparent;
  border-bottom: 1px solid hsla(210,18%,87%,1);
  height: .25em;
  padding: 0;
  margin: 24px 0;
  background-color: #d0d7de;
  border: 0;
}

input {
  font: inherit;
  margin: 0;
  overflow: visible;
  font-family: inherit;
  font-size: inherit;
  line-height: inherit;
}

[type=button],
[type=reset],
[type=submit] {
  -webkit-appearance: button;
}

[type=button]::-moz-focus-inner,
[type=reset]::-moz-focus-inner,
[type=submit]::-moz-focus-inner {
  border-style: none;
  padding: 0;
}

[type=button]:-moz-focusring,
[type=reset]:-moz-focusring,
[type=submit]:-moz-focusring {
  outline: 1px dotted ButtonText;
}

[type=checkbox],
[type=radio] {
  box-sizing: border-box;
  padding: 0;
}

[type=number]::-webkit-inner-spin-button,
[type=number]::-webkit-outer-spin-button {
  height: auto;
}

[type=search] {
  -webkit-appearance: textfield;
  outline-offset: -2px;
}

[type=search]::-webkit-search-cancel-button,
[type=search]::-webkit-search-decoration {
  -webkit-appearance: none;
}

::-webkit-input-placeholder {
  color: inherit;
  opacity: .54;
}

::-webkit-file-upload-button {
  -webkit-appearance: button;
  font: inherit;
}

a:hover {
  text-decoration: underline;
}

hr::before {
  display: table;
  content: "";
}

hr::after {
  display: table;
  clear: both;
  content: "";
}

table {
  border-spacing: 0;
  border-collapse: collapse;
  display: block;
  width: max-content;
  max-width: 100%;
  overflow: auto;
}

td,
th {
  padding: 0;
}

details summary {
  cursor: pointer;
}

details:not([open])>*:not(summary) {
  display: none !important;
}

kbd {
  display: inline-block;
  padding: 3px 5px;
  font: 11px ui-monospace,SFMono-Regular,SF Mono,Menlo,Consolas,Liberation Mono,monospace;
  line-height: 10px;
  color: #24292f;
  vertical-align: middle;
  background-color: #f6f8fa;
  border: solid 1px rgba(175,184,193,0.2);
  border-bottom-color: rgba(175,184,193,0.2);
  border-radius: 6px;
  box-shadow: inset 0 -1px 0 rgba(175,184,193,0.2);
}

h1,
h2,
h3,
h4,
h5,
h6 {
  margin-top: 24px;
  margin-bottom: 16px;
  font-weight: 600;
  line-height: 1.25;
}

h2 {
  font-weight: 600;
  padding-bottom: .3em;
  font-size: 1.5em;
  border-bottom: 1px solid hsla(210,18%,87%,1);
}

h3 {
  font-weight: 600;
  font-size: 1.25em;
}

h4 {
  font-weight: 600;
  font-size: 1em;
}

h5 {
  font-weight: 600;
  font-size: .875em;
}

h6 {
  font-weight: 600;
  font-size: .85em;
  color: #57606a;
}

p {
  margin-top: 0;
  margin-bottom: 10px;
}

blockquote {
  margin: 0;
  padding: 0 1em;
  color: #57606a;
  border-left: .25em solid #d0d7de;
}

ul,
ol {
  margin-top: 0;
  margin-bottom: 0;
  padding-left: 2em;
}

ol ol,
ul ol {
  list-style-type: lower-roman;
}

ul ul ol,
ul ol ol,
ol ul ol,
ol ol ol {
  list-style-type: lower-alpha;
}

dd {
  margin-left: 0;
}

tt,
code {
  font-family: ui-monospace,SFMono-Regular,SF Mono,Menlo,Consolas,Liberation Mono,monospace;
  font-size: 12px;
}

pre {
  margin-top: 0;
  margin-bottom: 0;
  font-family: ui-monospace,SFMono-Regular,SF Mono,Menlo,Consolas,Liberation Mono,monospace;
  font-size: 12px;
  word-wrap: normal;
}

.octicon {
  display: inline-block;
  overflow: visible !important;
  vertical-align: text-bottom;
  fill: currentColor;
}

::placeholder {
  color: #6e7781;
  opacity: 1;
}

input::-webkit-outer-spin-button,
input::-webkit-inner-spin-button {
  margin: 0;
  -webkit-appearance: none;
  appearance: none;
}

.pl-c {
  color: #6e7781;
}

.pl-c1,
.pl-s .pl-v {
  color: #0550ae;
}

.pl-e,
.pl-en {
  color: #8250df;
}

.pl-smi,
.pl-s .pl-s1 {
  color: #24292f;
}

.pl-ent {
  color: #116329;
}

.pl-k {
  color: #cf222e;
}

.pl-s,
.pl-pds,
.pl-s .pl-pse .pl-s1,
.pl-sr,
.pl-sr .pl-cce,
.pl-sr .pl-sre,
.pl-sr .pl-sra {
  color: #0a3069;
}

.pl-v,
.pl-smw {
  color: #953800;
}

.pl-bu {
  color: #82071e;
}

.pl-ii {
  color: #f6f8fa;
  background-color: #82071e;
}

.pl-c2 {
  color: #f6f8fa;
  background-color: #cf222e;
}

.pl-sr .pl-cce {
  font-weight: bold;
  color: #116329;
}

.pl-ml {
  color: #3b2300;
}

.pl-mh,
.pl-mh .pl-en,
.pl-ms {
  font-weight: bold;
  color: #0550ae;
}

.pl-mi {
  font-style: italic;
  color: #24292f;
}

.pl-mb {
  font-weight: bold;
  color: #24292f;
}

.pl-md {
  color: #82071e;
  background-color: #FFEBE9;
}

.pl-mi1 {
  color: #116329;
  background-color: #dafbe1;
}

.pl-mc {
  color: #953800;
  background-color: #ffd8b5;
}

.pl-mi2 {
  color: #eaeef2;
  background-color: #0550ae;
}

.pl-mdr {
  font-weight: bold;
  color: #8250df;
}

.pl-ba {
  color: #57606a;
}

.pl-sg {
  color: #8c959f;
}

.pl-corl {
  text-decoration: underline;
  color: #0a3069;
}

[data-catalyst] {
  display: block;
}

g-emoji {
  font-family: "Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";
  font-size: 1em;
  font-style: normal !important;
  font-weight: 400;
  line-height: 1;
  vertical-align: -0.075em;
}

g-emoji img {
  width: 1em;
  height: 1em;
}

.markdown-body::before {
  display: table;
  content: "";
}

.markdown-body::after {
  display: table;
  clear: both;
  content: "";
}

.markdown-body>*:first-child {
  margin-top: 0 !important;
}

.markdown-body>*:last-child {
  margin-bottom: 0 !important;
}

a:not([href]) {
  color: inherit;
  text-decoration: none;
}

.absent {
  color: #cf222e;
}

.anchor {
  float: left;
  padding-right: 4px;
  margin-left: -20px;
  line-height: 1;
}

.anchor:focus {
  outline: none;
}

p,
blockquote,
ul,
ol,
dl,
table,
pre,
details {
  margin-top: 0;
  margin-bottom: 16px;
}

blockquote>:first-child {
  margin-top: 0;
}

blockquote>:last-child {
  margin-bottom: 0;
}

sup>a::before {
  content: "[";
}

sup>a::after {
  content: "]";
}

h1 .octicon-link,
h2 .octicon-link,
h3 .octicon-link,
h4 .octicon-link,
h5 .octicon-link,
h6 .octicon-link {
  color: #24292f;
  vertical-align: middle;
  visibility: hidden;
}

h1:hover .anchor,
h2:hover .anchor,
h3:hover .anchor,
h4:hover .anchor,
h5:hover .anchor,
h6:hover .anchor {
  text-decoration: none;
}

h1:hover .anchor .octicon-link,
h2:hover .anchor .octicon-link,
h3:hover .anchor .octicon-link,
h4:hover .anchor .octicon-link,
h5:hover .anchor .octicon-link,
h6:hover .anchor .octicon-link {
  visibility: visible;
}

h1 tt,
h1 code,
h2 tt,
h2 code,
h3 tt,
h3 code,
h4 tt,
h4 code,
h5 tt,
h5 code,
h6 tt,
h6 code {
  padding: 0 .2em;
  font-size: inherit;
}

ul.no-list,
ol.no-list {
  padding: 0;
  list-style-type: none;
}

ol[type="1"] {
  list-style-type: decimal;
}

ol[type=a] {
  list-style-type: lower-alpha;
}

ol[type=i] {
  list-style-type: lower-roman;
}

div>ol:not([type]) {
  list-style-type: decimal;
}

ul ul,
ul ol,
ol ol,
ol ul {
  margin-top: 0;
  margin-bottom: 0;
}

li>p {
  margin-top: 16px;
}

li+li {
  margin-top: .25em;
}

dl {
  padding: 0;
}

dl dt {
  padding: 0;
  margin-top: 16px;
  font-size: 1em;
  font-style: italic;
  font-weight: 600;
}

dl dd {
  padding: 0 16px;
  margin-bottom: 16px;
}

table th {
  font-weight: 600;
}

table th,
table td {
  padding: 6px 13px;
  border: 1px solid #d0d7de;
}

table tr {
  background-color: #ffffff;
  border-top: 1px solid hsla(210,18%,87%,1);
}

table tr:nth-child(2n) {
  background-color: #f6f8fa;
}

table img {
  background-color: transparent;
}

img[align=right] {
  padding-left: 20px;
}

img[align=left] {
  padding-right: 20px;
}

.emoji {
  max-width: none;
  vertical-align: text-top;
  background-color: transparent;
}

span.frame {
  display: block;
  overflow: hidden;
}

span.frame>span {
  display: block;
  float: left;
  width: auto;
  padding: 7px;
  margin: 13px 0 0;
  overflow: hidden;
  border: 1px solid #d0d7de;
}

span.frame span img {
  display: block;
  float: left;
}

span.frame span span {
  display: block;
  padding: 5px 0 0;
  clear: both;
  color: #24292f;
}

span.align-center {
  display: block;
  overflow: hidden;
  clear: both;
}

span.align-center>span {
  display: block;
  margin: 13px auto 0;
  overflow: hidden;
  text-align: center;
}

span.align-center span img {
  margin: 0 auto;
  text-align: center;
}

span.align-right {
  display: block;
  overflow: hidden;
  clear: both;
}

span.align-right>span {
  display: block;
  margin: 13px 0 0;
  overflow: hidden;
  text-align: right;
}

span.align-right span img {
  margin: 0;
  text-align: right;
}

span.float-left {
  display: block;
  float: left;
  margin-right: 13px;
  overflow: hidden;
}

span.float-left span {
  margin: 13px 0 0;
}

span.float-right {
  display: block;
  float: right;
  margin-left: 13px;
  overflow: hidden;
}

span.float-right>span {
  display: block;
  margin: 13px auto 0;
  overflow: hidden;
  text-align: right;
}

code,
tt {
  padding: .2em .4em;
  margin: 0;
  font-size: 85%;
  background-color: rgba(175,184,193,0.2);
  border-radius: 6px;
}

code br,
tt br {
  display: none;
}

del code {
  text-decoration: inherit;
}

pre code {
  font-size: 100%;
}

pre>code {
  padding: 0;
  margin: 0;
  word-break: normal;
  white-space: pre;
  background: transparent;
  border: 0;
}

.highlight {
  margin-bottom: 16px;
}

.highlight pre {
  margin-bottom: 0;
  word-break: normal;
}

.highlight pre,
pre {
  padding: 16px;
  overflow: auto;
  font-size: 85%;
  line-height: 1.45;
  background-color: #f6f8fa;
  border-radius: 6px;
}

pre code,
pre tt {
  display: inline;
  max-width: auto;
  padding: 0;
  margin: 0;
  overflow: visible;
  line-height: inherit;
  word-wrap: normal;
  background-color: transparent;
  border: 0;
}

.csv-data td,
.csv-data th {
  padding: 5px;
  overflow: hidden;
  font-size: 12px;
  line-height: 1;
  text-align: left;
  white-space: nowrap;
}

.csv-data .blob-num {
  padding: 10px 8px 9px;
  text-align: right;
  background: #ffffff;
  border: 0;
}

.csv-data tr {
  border-top: 0;
}

.csv-data th {
  font-weight: 600;
  background: #f6f8fa;
  border-top: 0;
}

.footnotes {
  font-size: 12px;
  color: #57606a;
  border-top: 1px solid #d0d7de;
}

.footnotes ol {
  padding-left: 16px;
}

.footnotes li {
  position: relative;
}

.footnotes li:target::before {
  position: absolute;
  top: -8px;
  right: -8px;
  bottom: -8px;
  left: -24px;
  pointer-events: none;
  content: "";
  border: 2px solid #0969da;
  border-radius: 6px;
}

.footnotes li:target {
  color: #24292f;
}

.footnotes .data-footnote-backref g-emoji {
  font-family: monospace;
}

.task-list-item {
  list-style-type: none;
}

.task-list-item label {
  font-weight: 400;
}

.task-list-item.enabled label {
  cursor: pointer;
}

.task-list-item+.task-list-item {
  margin-top: 3px;
}

.task-list-item .handle {
  display: none;
}

.task-list-item-checkbox {
  margin: 0 .2em .25em -1.6em;
  vertical-align: middle;
}

.contains-task-list:dir(rtl) .task-list-item-checkbox {
  margin: 0 -1.6em .25em .2em;
}

::-webkit-calendar-picker-indicator {
  filter: invert(50%);
}