Need help adding and editing theme file

My co-blogger who has done all the work customizing our theme, is largely unavailable and I’m hoping to get some help adding something to our right sidebar:

https://transcendentalfloss.com

As you can see, the sidebar is split into sections, each section has it’s own colored background, and each section automatically expands or shrinks to adjust to how much content is being displayed.

I’d like to add a new section under the orange section that has the Jewish Buddhism Tip in it. In this new section, I plan on adding this embedded javascript tag:

https://www.kirkdorffer.com/pnw/pnwjavascripttag.shtml

We have a test version of our site where I added this to that bottom orange section, just to see what it would look like there, and that’s how I decided I needed to add a new section.

Here’s the test site:

https://blogcopy.transcendentalfloss.com/

For the background color of this section, I’d like to use the same light gray as in the section toward the top that starts with “Cascadian Links”. I’m not sure how the javascript tag will display on that color background, since, with the orange background, there’s a combination of white, dark gray, and light gray text. (I can see from the website offering this script that there are CSS definitions you can play around with, but that’s over my head.)

Now, I know how to look in the Theme Editor and do some light editing of the files we do have. But I’ve never added a new theme file. I assume it involves using an ftp client, but the only experience I have using ftp is to upload photo, music, and video files that I want to link to.

Sorry this is so long, but I thought the helpers out there would appreciate the detail so that they don’t have to ask a ton of follow-up questions. (Oh, and yeah, I was bad yesterday and bumped my first thread here after only an hour. I hope that hasn’t burned a bridge.)

Thanks!