My fault. I did not do enough research. I have just found (running Xfce4 as a web browser), within the browser menu, in Edit>Prefences>Content, the box 'Allow pages to choose their own fonts'. I set it to 'Do not allow'. Now I can set the font size for sourceforge pages at will.
Only that when I am in LQ, their excellent fonts are disabled and replaced by mine. A pity. So I am forced to switch back and forth the fonts settings in Preferences>Content.
About a link to the fonts, that would be much beyond the present state of my knowledge in Linux, or in any other O.S. Thanks a lot for your reply.