If you are really required app.config and nothing else (it could be any kind of configuration file holding current options which could be saved and loaded during runtime any time, in contrast to app.config), you need to read this:
https://msdn.microsoft.com/en-us/library/2tw134k3%28v=vs.140%29.aspx[
^].
I hope, application of fonts and other properties won't be a problem for you.
Above, I explained you the benefit of using not configuration file, but something else. Another problem with app.config is: if this file can be used for some "serious" application tune-up, so contaminating it with many custom sections would be pretty bad. This is another reason for saving the UI options in a separate file. Most usually, settings are used, but I think they are obsolete, not flexible enough, so I never use this feature, use my own file instead. Please see:
https://msdn.microsoft.com/en-us/library/ms733127%28v=vs.110%29.aspx[
^].
—SA
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