I replaced all the fixed font-sizes with relative font-size. text becomes a lot more readable and is resizeable (like it should be).
this css file is for the default template.
in your Joomla-template you got to set font-size for body on 0.8em.
location:
componentscom_fireboardtemplatedefault/forum.css

posting the css-file was not allowed (?), so I made it a .txt, so you need to copypaste this into your forum.css.

make a backup first.