Reduce White Space in HTML5 Output - Robohelp 2015

There's an excessive amount of blank space/white space in my HTML 5 output (I'm using a slightly customized version of the Indigo output.) I'm sure there's an adjustment in a .css somewhere but I've been unable to track down where that adjustment can be made:

I'd prefer the center-content window to be wider, taking up most of the primary screen. Anyone know how to fix this?