You may override the ShellWindowMorph default #textStyle by the following one:
textStyle
" Answer a font for the text morph"
^ TextStyle default
The window should have a way to set the font directly from the shell menu (the top right arrow) or from the Settings Browser. You may modify any morph property by bringing the "Halo" menu over the morph you are interested (Alt+Shift+Left Click under MS Windows) and select the red icon. Then select Debug -> Explore morph and set the text style directly from the Explorer:
self textMorph setTextStyle: TextStyle default