In the same folder as index.html and play.html is style.css, which contains the parameters for the "gameport" div style:
#gameport
{
background: white;
bottom: 0px;
left: 15.4em;
line-height: 1.4;
margin: 0px;
position: absolute;
right: 0px;
top: 0px;
}
"left", "right", and "top" refer to the margins around the division. The line that says "right: 0px;" is the one that says there should be zero pixels between the edge of the gameport and the edge of the browser window. You can change it to a different number of pixels, a number of ems (an "em" is a unit of distance relative to the size of a font...and I forget how they work), or a percentage.
So, if you change that line to "right: 15.4em;", the right margin will be the same size as the left margin, and that might be what you're looking for—but you can try out all kinds of stuff.