How to Make Previous Post Arrow Align Left

I would like to change the "previous post" arrow in the gray rectangle that floats along the left as you read an article on I would like it to run along the lefthand side of the browser (just like the "next post" arrow runs always all the way on the righthand side of the browser window).

I believe the css that controls the position of the previous post arrow is as follows:

.prevBtn {
margin-left: -99px;

But rather than setting it to a specific number of pixels to the left of the article body, I'd like to set it to always run at the outermost edge of the browser window (like the "next post" arrow does).

If it is helpful, I have granted admin access to my website through the wpmu plugin.

Thanks for your help!