Unwanted comma concatenated on highlight

[lastboy]

When running more than one highlight command the style tag blocks are separated with unwanted comma within the generated HTML output

I coded my custom highlight module that exported the css:

module.exports.css = function() {
    return '<style>' + require("fs").readFileSync(__dirname + '/override.css') + '</style>';
};

I added my custom highlight to the cmd:
generate-md --layout mixu-page --input ./core --output ./core/ --highlight-hljs mds-hljs --highlight-cathl ./cathl.js

In the HTML output I got the first mds-hljs style tag block and after that my custom style tag block but separated with unwanted comma.