I am wondering (paging eigen!) whether there are other themes available out there we could add on. There are only three right now. I have to believe there are more out there. I just switched over to the Dani theme - which does have that characteristic where quoted text is the same font size as unquoted text. Maybe I'm wrong but when I think of popular software like Wordpress (granted, it's not forum software) there are tons of themes to choose from.
OK a quick google search does verify that many themes are available. So perhaps that is something we could do.
There are a ton. I mentioned in the mobile theme thread that I'm perfectly happy to add any themes people find and want (assuming they work with the current software version), but I don't want to spread myself too thin in troubleshooting them.
That said, if you want to take on some of the coding details for a new theme (or two, or three) we can definitely do it!
Some of the issues (like the top login not working on the original theme) we're likely to encounter with new themes we add, so some may be a "available to use as-is" standpoint.
The browser for themes is here:
https://custom.simplemachines.org/themes/Any of the ones there are pretty easy to set up for us to use- might take half an hour if they work as promised.
On the original discussion on font size: It's not particularly easy. I did some reading in the Simple Machines support forums, and it looks like it would require overhaul of some of the basic code for the site, and might have unintended consequences. It also has the side effect of making it hard/impossible for anyone that currently has their browser zoomed (i.e., Polly), and I know I wouldn't want the default font any larger than it is.
You can definitely set browser based font sizes that carry over here- I have a computer at home that sits up on a mantle as a media server, and I used Firefox to dramatically increase the min font size in the browser, and that works with these forums as well as any other site.