Github markdown theme for webstorm's markdown preview. just use this url in "Settings -> Languages & Frameworks -> Markdown -> Custom CSS -> Load from URI": https://gistcdn.githack.com/alirezamirian/aa06088df9310a4fccccb369a74815ac/raw/3004f7d4d3e1c0263b546aead7310fd8a34d1b12/webstorm-markdown-preview-github-style.css

webstorm-markdown-preview-github-style.css

body {
  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;
  color: #24292e;
  background-color: #fff;
}

body kbd {
  display: inline-block;
  padding: 3px 5px;
  font: 11px "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;
  line-height: 10px;
  color: #444d56;
  vertical-align: middle;
  background-color: #fafbfc;
  border: solid 1px #d1d5da;
  border-bottom-color: #d1d5da;
  border-radius: 3px;
  box-shadow: inset 0 -1px 0 #d1d5da;
}
body::before {
  display: table;
  content: "";
}
body::after {
  display: table;
  clear: both;
  content: "";
}
body > *:first-child {
  margin-top: 0 !important;
}
body > *:last-child {
  margin-bottom: 0 !important;
}
body a:not([href]) {
  color: inherit;
  text-decoration: none;
}
body .absent {
  color: #cb2431;
}
body .anchor {
  float: left;
  padding-right: 4px;
  margin-left: -20px;
  line-height: 1;
}
body .anchor:focus {
  outline: none;
}
body p,
body blockquote,
body ul,
body ol,
body dl,
body table,
body pre,
body details {
  margin-top: 0;
  margin-bottom: 16px;
}
body hr {
  height: 0.25em;
  padding: 0;
  margin: 24px 0;
  background-color: #e1e4e8;
  border: 0;
}
body blockquote {
  padding: 0 1em;
  color: #6a737d;
  border-left: 0.25em solid #dfe2e5;
}
body blockquote > :first-child {
  margin-top: 0;
}
body blockquote > :last-child {
  margin-bottom: 0;
}

h1,
h2,
h3,
h4,
h5,
h6 {
  margin-top: 24px;
  margin-bottom: 16px;
  font-weight: 600;
  line-height: 1.25;
}
h1 .octicon-link,
h2 .octicon-link,
h3 .octicon-link,
h4 .octicon-link,
h5 .octicon-link,
h6 .octicon-link {
  color: #1b1f23;
  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 {
  font-size: inherit;
}

h1 {
  padding-bottom: 0.3em;
  font-size: 2em;
  border-bottom: 1px solid #eaecef;
}

h2 {
  padding-bottom: 0.3em;
  font-size: 1.5em;
  border-bottom: 1px solid #eaecef;
}

h3 {
  font-size: 1.25em;
}

h4 {
  font-size: 1em;
}

h5 {
  font-size: 0.875em;
}

h6 {
  font-size: 0.85em;
  color: #6a737d;
}

ul,
ol {
  padding-left: 2em;
}
ul.no-list,
ol.no-list {
  padding: 0;
  list-style-type: none;
}

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

li {
  word-wrap: break-all;
}

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

li + li {
  margin-top: 0.25em;
}

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 {
  display: block;
  width: 100%;
  overflow: auto;
}
table th {
  font-weight: 600;
}
table th,
table td {
  padding: 6px 13px;
  border: 1px solid #dfe2e5;
}
table tr {
  background-color: #fff;
  border-top: 1px solid #c6cbd1;
}
table tr:nth-child(2n) {
  background-color: #f6f8fa;
}
table img {
  background-color: transparent;
}

img {
  max-width: 100%;
  box-sizing: content-box;
  background-color: #fff;
}
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 #dfe2e5;
}
span.frame span img {
  display: block;
  float: left;
}
span.frame span span {
  display: block;
  padding: 5px 0 0;
  clear: both;
  color: #24292e;
}

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: 0.2em 0.4em;
  margin: 0;
  font-size: 85%;
  background-color: rgba(27, 31, 35, 0.05);
  border-radius: 3px;
}
code br,
tt br {
  display: none;
}

del code {
  text-decoration: inherit;
}

pre {
  word-wrap: normal;
}
pre > code {
  padding: 0;
  margin: 0;
  font-size: 100%;
  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;
  color: inherit!important;
  border-radius: 3px;
}

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

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

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

tr {
  border-top: 0;
}

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