I would start looking at TextCtrlLogger::UpdateSettings() in file loggers.cpp in folder src/sdk.
Edit: If you wish to add a spot in the GUI to change this font, I suggest here:
Settings -> Environment
Select "View" in left panel
FYI: I started searching on the info on this location in the xrc files then used the info in the xrc to go to a cpp file and to the CB config setting to another cpp file and so on.
Edit2: This line is my guess
wxFont default_font(size, fixed ? wxFONTFAMILY_MODERN : wxFONTFAMILY_DEFAULT, wxFONTSTYLE_NORMAL, wxFONTWEIGHT_NORMAL);
Tim S.