Ash here from the Shopify team. This is a great question! While this is not a change you'll make where most theme customizations are made, it's absolutely possible to do.
Your first step is to head over to the Online Store> Themes section of Shopify, and use the "Actions" menu for the theme in question to access your themes language editor. Here is some more information on the language editor for themes, including how to access and use it. Once you're in the language editor, you should see some tabs along the top which refer to different sections of the store; General, Blogs, Cart, Collections, etc. Click on the "Collections" tab.
Once in there, you should be able to see the placeholder text for when there are no product matches for a collection. You can change the placeholder text and save the page to change what customers see on your live site when visiting an empty collection. Here's a screenshot to help visualize:
I hope this is helpful, and feel free to reply back here if you have any further questions or have any issues tracking down this section and making the desired change.
My pleasure, @TierraAb0de!
So the method I described above will allow you to change the wording on certain areas of your site. Adding a new button to your site would be a bit different.
Usually an application that is used for adding buttons to a site will work in one of two ways. One way, is once the application is installed, there will be options within the application to use its different features, such as adding a button to a specific page. The other way, is the app providing some code for you copy and paste into your theme to apply features to the page(s) of your choosing. Either way, I wouldn't expect this to interfere with the change in the wording you make in the themes language editor.
I hope this helps, and let me know if you need further clarification on any points!