[CYS on Core] Fix the failed to load resource error in the CYS whenever the current active theme is not TT4

[nefeline]

Issue Description:

Whenever you access the CYS flow for the very first time and your current active theme is not TT4, it is possible for the following error to come up when clicking on "Start designing" > "Design a new theme" modal:

Failed to load resource: the server responded with a status of 404 ()

The resource that is failing to load is:

wp-json/wp/v2/global-styles?context=edit&_locale=user

Steps to reproduce

1. Make sure you have the WooCommerce Beta Tester plugin installed and activated
2. Under Tools > WCA Test Helper > Features, make sure customize-store is enabled
3. Under Tools > WCA Test Helper > Tools, click on Reset Customize Your Store
4. Make sure your current active theme is TT1
5. Head over to WooCommerce > Home > Customize your store
6. Click on "Start designing" and then on "Design a new theme" when the modal opens
7. See the following error: