The instructions to add custom JS or CSS to Catalog are documented. However, I had a hard time finding instructions to do that for Canvas.
I'm posting this here so others can find it in the future. Thanks to Stefanie Sanders for her help.
Test this in your beta instance first.
1. Login as Admin to Canvas
2. Click on Themes
3. Scroll through the list and open the current theme in Theme Editor.
4. Click on the Upload tab
5. Click on View to view existing JS or CSS files. Copy to an editor like Notepad. (VERY IMPORTANT STEP: You do not want to lose existing js/css)
6. Add your custom code to that Notepad file and save locally. JS files are saved with an extension of .js and css files with an extension of .css
7. Use the Select button to navigate the file system and upload the file to Canvas.
8. Save the Theme (Give it a name if it does not have one already).
9. Apply the Theme.
10. Test, test, test