As with overriding the default system phrases and emails I usually recommend leaving the system defaults alone and editing them at the application level.
It's the same amount of work to override at the application level vs setting at the system level. The differences comes when you want to add another program and have conflicts that need addressing since you've edited the defaults at the system level.
The same applies even stronger to submission / evaluation forms.
The guidance I give is something like this:
- If you want your customization to apply to all pages and programs on your website, add them to your Public Settings -> Themes.
- If you want your customization to apply to an individual program / form, add them to Round Settings > Submission/Evaluation Form > Developer Settings > Scripts.
Overall, making changes at the application level is safer as they'll be applied in a more controlled manner as opposed to running on everyone of your programs / pages. The only time you'll want to add it to your Theme is if you want the change to apply to every program and page on your website.