Before answering your questions, I’ll let you know that we’ve just introduced a new setting on the user profile called Text Size. Each user from now on will be able to enlarge or reduce the character of the site as prefer. See your for example your profile https://meta.discourse.org/u/MichelleBasey/preferences/interface
We have made a lot of changes since 2014 … Now you have to choose “where” you want to make changes, only on Desktop, only on Mobile or in Common(both Desktop and Mobile) and then choose what to change, in your case the CSS tab to change the size of the font. If you need to change the font itself you can still use the @import url(); and the link rel="stylesheet" href="#"> mode, but it is easier to upload the font directly in the site assets following this guide Include assets (e.g. images, fonts) in themes and components.
The font is always the same, Helvetica (Arial,sans-serif). We only bump up the size 1px.
This depends on the different browsers not by us, each browser is set differently, you should visit the various settings of each browser to make the font the same size (at least).