Sorry for the inconvenience, but this feature of styling read more in joomla is not available in TemplateToaster. We are working on this and try to provide it in the next major release of Templatetoaster.
Meanwhile you can add the following custom css at TemplateToaster -> File -> Prefrences -> Custom css ->
p.readmore > a {
background: none repeat scroll 0 0 hsl(0, 0%, 100%);
border: 1px solid hsl(0, 0%, 0%);
border-radius: 5px;
padding: 5px;
}