I did create my own CSS file to change the look and feel of the modal. See attached screenshots. If anyone is interested in getting this modal css, no problem. Just let me know. I did not change the js file.
I do miss one thing in the frontend modal: The title of the modal. We use a translation for PLG_SYS_SESSIONKEEPER_MODALTITLE, but this string/titel does not show up in the modal header. Maybe there is something missing in the js?