Result tab pane is highly customizable as it is based on CSS. To change the font:
1) Right click into editor pane (not in output pane) and select Preferences...
2) Select the Result tab and in the Style sheet for text (HTML) results select Custom and select Launch Editor button. This will launch LINQPad Stylesheet Editor.
3) The top pane holds the read only Base Styles, so you can examine elements defined there in order to customize some of them.
body tag has only one font-family defined: Verdana,and I wanted to add "Lucida Console" monospaced font instead.
4) In the Customizations tab add CSS for body tag overriding or adding parts you need, for instance
body{
font-family: "Lucida Console", Verdana;
background: cornsilk;
}
5) Select Apply Changes button. This will immediately apply the changes and turn the button disabled, so close the LINQPad Stylesheet Editor and select OK button to finish.
Next time you select the Execute button will take CSS changes into account.

Also in query window would be good.