When your admin theme is different from your frontend theme, the icms_view_Tpl class will always take the value in $icmsConfig['theme_set'] to identify where to look for eventual theme overrides (check out line 70, where the compile_id is constructed.
This could be the reason why the template overrides in a admin theme don't seem to work.
I think this information cannot be determined by the icms_view_Tpl class on its own, it needs input from the calling class.
When your admin theme is different from your frontend theme, the icms_view_Tpl class will always take the value in $icmsConfig['theme_set'] to identify where to look for eventual theme overrides (check out line 70, where the compile_id is constructed. This could be the reason why the template overrides in a admin theme don't seem to work. I think this information cannot be determined by the icms_view_Tpl class on its own, it needs input from the calling class.