Mark Text allows you to modify the appearance of the document that you want to export. By default we provide three themes: Academic, GitHub and Liber (writing theme).
You can install a theme by copying the .css file to themes/export/ directory inside the application data directory location but you may need to restart Mark Text to detect the theme.
Mark Text use the GitHub theme as basic style that is always available. A custom theme can add additional styles but have to overwrite the GitHub style to make changes such as font family or the underling for heading. You can see all predefined styles here. An example for custom themes can be found here and here.
A theme currently only have a name (A-z0-9 -) that is defined by a CSS comment at the first line like:
/** Liber **/
.markdown-body {
/* ... */
}