Changing Sidebar Width in Modularity Lite

Oops… I put this as a response to a similar thread earlier, but it would have been better to put it here, I guess. I’m afraid this is not brief, but it is as descriptive as I know how to be. And yes, I’ve looked all over for the solution, but have none that are in a language I understand.

This is my first time (now second) posting here, so please forgive my lack of knowledge. Here’s my site so far, in case it matters: https://mimitopia.com/ It’s functioning well so far, but I’d love to solve just this one issue.

I’ve been searching all over trying to figure out how to change the size of the sidebar in Modularity Lite, and have seen many posts including this one. But none of them have helped me. Right now I’m using the full-width option without a sidebar because the sidebar takes up so much space that there’s no room for my images. Since my website will be for selling art prints, I really need space for the images. However, I find there’s something “missing” by not having a sidebar. I’d like to have one, just skinnier. It seems weird to me to have that wide of a sidebar on a theme that’s meant for visual stuff. Still, I love the theme in every other way, so I’m not complaining… much.

I do not have a page in editor called “screen.css”. I have style.css of course, and supposedly I’m to put this code:

.span-7 { width: 650px; }
.span-15 { width: 590px; }

Presumably I get to specify how many pixels wide each field is. But where exactly does the code go? Is there a line somewhere in style.css within which I paste the code? Or do I just drop it anywhere on the page, pinch my nose and dive in? I’m horribly inept at code-language and really need someone to tell me exactly where to put these things.

Okay, that’s a really long question for what I’m sure is a very simple issue. But simple or not, it has perplexed me for many hours, so I’m finally reaching out ??

Thanks in advance!