Hey guys ... that must have been a typical nerd problem. I've inspected all the bits within the CSS of the kepub files. Compared them with others that I bought from the Kobo store. .... looked for failures in the translation plugins.
.... however, being relatively new to Kobo, I completely overlooked the normal 'font' menu on top of the Reader. I'm slightly embarrassed
But after all, you guys still kept the patience to help me
And I'll definitively stop hacking line-height and font-size attributes, know that I know that they have little effect onto the device.
Here are the improved results ... and a screenshot of the menu for other beginners like me
