If you guys right-click on the Kod.app and Show Package Contents then navigate to the Contents/Resources/style folder you'll see a default.css file. The comments at the top of the file read:
/*
* This is the default style of Kod. If you wish to modify this to your liking,
* make a copy and put it in ~/.kod/mystyle.css then tell Kod where to find it
* by setting a configuration key:
*
* $ defaults write se.hunch.kod style/url ~/.kod/mystyle.css
*
* You might need to restart Kod for changes to take effect. Then, simply open
* your ~/.kod/mystyle.css in Kod -- when edited and saved Kod will
* automatically reload the style, thus you see the effect of your alterations.
*/
I imagine this is documented in the wiki someplace but editing this file or following the directions and creating a new file (like ~/.kod/mystyle.css as described in the comments) will allow you to change the font-size, background color and any other setting that is supported by the CSS engine that dictates how the app renders each part of each style. If you don't know CSS there are plenty of tutorials on the web that can guide you. Even a few educated guesses will go a long way to getting the editor to do what you'd like.
Enjoy and I hope this helps,
Gabriel