I'm using Opera 12.x on 2 older systems, Win7 and OSX.6.8, Snow Leopard, and like their stability and features. And I use the native Opera Source viewer/editor to keep customized versions of web pages locally -- can edit the html, instantly view my changes in another tab, even Save the edited code to local directory. I prefer this to using an external editor, as it avoids switching between apps, and a few extra clicks.
But on OSX, the Source viewer font size is tiny and not adjustable. In the Windows app, I can change the font size using Preferences/Advanced/Fonts and edit the "email compose" font. I've tried all sorts of things in opera:config, editing some of the .css files inside the app package: Contents/Resources/Styles, etc. -- nothing helps! Hopefully someone here can.