locking css code and column width

Hello!

I’m making adjustments on a WordPress website that someone else built. It’s a custom theme and I’m running WordPress 3.9.1.

My customer wanted the graphs on many pages to be bigger. The theme is built so that the column would have to be made wider in order for the graph to be bigger. Did I mention that it’s a custom theme? I went in to “Appearance” and “Edit” and there was a button to click to make What I suspect is the CSS code “editable”. Well, I looked at it and decided I didn’t want to mess with it at this time.

I have two questions:

1. Where do I change the column width? My understanding is it’s preferred to make a child theme (sometimes?) before making changes directly in the CSS code. Correct?

2. Also I wanted to unclick whatever button it was that made it “editable” and secure it again. How do I do that?

here’s a link to one of the pages that has a graph (at the bottom).

http://2b8.7a4.myftpupload.com/products/ek8521-sapphire-window/