There is no way to change it forum Layouts
The forum layouts are changed for each category (top level forum) by editing that category and choosing one of the four forum layouts:
Here is the documentation: https://wpforo.com/docs/root/categories-and-forums/forum-layouts/
The screenshot you're showing is the wpForo Theme management page, there is no other theme yet. Currently, there is only one Classic theme with the four layouts. You can set forum layouts as I explained above.