Theme looks weird after removing plugins

I am using the SMT theme by mooninwell. I was asked to do some updates on a website, truenorthfarmpa.com. I was doing well, clients were pleased, but one day I decided to start cleaning out some of the plugins which didn’t appear to be used for anything. Unfortunately, it looks like I accidentally removed some that were still being used prominently on the page.

Now the image slider on the front page doesn’t work, the background image won’t display, and even the top menu displays incorrectly, as though it’s running on different css than it used to.

What’s going on, and how do I fix it? I’d love to just return whatever Nivo slider plugin was making the slider work, but there are a lot of those and I can’t figure out which one would bring it back.

Interestingly, when I change to a different theme and then use “preview” mode for the SMT theme, it looks perfectly normal. I even took screenshots to demonstrate the “right” and “wrong” look for the site. If you’d like to see them, I’ll try to get them to you however I can.

I considered reinstalling the theme, but it was already installed by somebody else, I don’t have a clean copy of it, and I haven’t been able to reach the vendor for help.