The ROOter web interface has three different looking GUI themes available for it, Bootstrap, Material and Dark Matter.
The Material theme, which is the default, has the menus at the side and is a modern looking GUI.
The Bootstrap theme is the theme onced used by ROOter.
The Dark Matter theme is similar to Material but has different colors and look. This theme is not available on routers using the 4meg images.
To switch between these themes you go to the System menu and the System< submenu and click on the Language and Style tab. In the Design dropdown box choose the theme you wish to use.
Click on Save & Apply. The theme will change when you go to a different part of the web interface.