use --jp-content-font-family for rendered HTML

packages/rendermime/style/index.css

@@ -93,6 +93,7 @@
   color: var(--jp-content-font-color1);
   font-size: var(--jp-content-font-size1);
   line-height: var(--jp-content-line-height);
+  font-family: var(--jp-content-font-family);
   /* Give a bit more R padding on Markdown text to keep line lengths reasonable */
   padding-right: 20px;
 }