Because I really wanted the old look back: Font Finder shows the new font as indeed Open Sans, while looking at the old font (still used in the post creation window) it apparently was Lucida Grande, Geneva, Arial, sans-serif.

So I used InkPanther's solution above and set the old font/size, plus a different size on the forum/general page for topics (at least that's what I recall it looked like):

Thank you! So, the font did change, I thought I was crazy and just imagining it.

Was about to start a thread asking if this was the case when GOG's search returned this.
You recall correctly, the font size on the page for topics was smaller than the rest.
Before
https://web.archive.org/web/20170202072230/https://www.gog.com/forum/general#1486019884
After
https://web.archive.org/web/20170615235443/https://www.gog.com/forum/general#1497570687

Your solution makes it close to what it was originally, however does anyone know how to make the page links for multipage threads "[p: 1 - n]" appear smaller too? At the moment they are too big.

EDIT:

got it. it's the .pages

So:

Did you endure the awful look all this time? And thanks for the wayback machine link, you are right, the .pages should be smaller than I have it now. Going to set that right away.