After a discussion with our front-end developers we came to the conclusion that the contrasts between the themes arise from the different UI design choices that guided each theme's creation. The main distinction in terms of the group rows comes from the fact that in some themes when the group row is not expanded, its arrow points down and in other themes it points right. The expanded state of the group row's arrow is another thing that differs due to specific design choice for each theme. 

That said, if you want to change the orientation of the arrows inside the group row for a specific theme, you can create a custom style. Of course, feel free to contact us by creating a forum thread/ticket, if you have any questions. 

Regards,
Vladimir Stoyanov
Progress Telerik