We investigated the webpage you have provided and find out that your theme is force adding those borders to your tables. If you use the CSS code I provided below at Additional CSS section of theme customization menu of your theme, issue will be fixed.
Thanks for the reply .. I always try to stay away from css-files, as I am afraid to skrew up things completely …
However, I added the line you supplied in the custom.css, nothing changed. I added it in the layout.css, nothing changed. There are many css-files: custom, base, grid, rtl, admin-preview, shortcodes, style.css, … Which one to use???
You should add custom CSS code through Additional CSS menu of the theme customize menu. You can reach that menu through Themes > Customize > Additional CSS from admin menu. Here are screenshots that will guide you to that functionality.
You should add all kinds of custom CSS code through Additional CSS option of theme customization. You can reach that option through Appereance > Customize > Additional CSS from admin menu. Here are some screeenshots that will guide you through those menus.