The Gray theme icons should be available in the gray theme. But they are used a bit differently in Ext.NET 7. If you show a simple scenario in v5- where you use a icon that you want to replicate in v7, we can show you how to do it.
Basically you'd be using
iconCls with a reference to the icon, instead of just
Icon="Add" in previous versions. As it was a complete rewrite of the Ext.NET engine to .NET Core (.NET 5), we simply couldn't implement some syntax sugar previous versions uses for that and other features. Besides, the newer themes (Spotless, Material, Triton, Graphite, Aria) uses font icons, so these "old themes" icons can't be automatically mapped between the themes.
But again, the old themes are still there, and their icons too. Just tell us what your scenario is and we'll tell you what to do. Then you probably will be able to expand the usage to other components where you use them.
Looking forward to your follow-up!