That particular thing you're editing in the controlStylesconfig isn't the alerts tab. It's that popup alert for when the ETR expires on an incident. There is no setting that I've ever seen that controls the font size of all of Explorer.
To change the font size on the entire grid? I think you'd have to change the font size per PC and per user via the Display settings in the Control Panel.
I have tried it via the display settings but that of course affects the entire system so that was not the solution for us.
I guess changing the appearances on each ControlStyle section individualy will be the solution.