it may happen that keys left and right don’t work as expected, showing two columns instead of one. You can try to resize the window.
keeping updated your custom css (in Calibre viewer or in markdown-viewer browser extension)
You can link within the css (in Calibre it is within the file “viewer-webengine.json”, in markdown-viewer browser extension is the css file you yourself have chosen) your external css file, with something like
@import url(http://localhost/path-to-your-css-file/your-css-file.css)
Provided you have a apache/php server in localhost, of course.