Lem, you have got it right. I just managed to miss the Advanced appearance settings. Now I have everything adjusted the way I want it, except the fonts on the bookmark bar. But when you use folders, the fonts in the dropdown menus can be adjusted. So not a problem.

Thanks for answer Lem. Readability and and font size are very good in Opera 21, except for bookmarks and menus. I am an experienced user of both windows and linux, and if there is something I cannot do, it is either impossible, or for very advanced users. In this case I suspect there is no solution to the problem, except using both 21 and an older version on the same computer. I have older versions of Opera on one win 8.1 laptop and one Linux laptop, so I will manage. Using any other browser than Opera is, of course, out of the question.

I have just installed Opera 21, and the only thing I am not satisfied with is the font size. Will going back to 20 increase the font size significantly? I went from an older version to 21, so I have not tried 20.
Of course the developpers should fix this as soon as possible. The solutions mentioned in this thread is not possible in my version of Win 7.