Since upgrading to 4.7.3, sadly, only custom CSS files are shown in the Theme Editor side window now.

Core and other applications CSS files are no longer showing or can be edited without going into designer mode.

This is correct and as per design. It was changed some time ago, and has been discussed at length here

https://invisioncommunity.com/forums/topic/468027-css-questionor-disappointment/#comment-2897150