OneUI has two components.  The Framework and the look and feel.

 

The Framework (best example is the Layout Control from the Extension LIbrary) is used for a consistent presentation. 

 

The look and feel is controlled by the underlying CSS. 

 

To have different content in the left navigation for the layout then create a custom control that uses the layout control and enable the left panel area. 

 

Create individual custom controls for the content to be displayed in the left area. 

 

Create individual XPages, add the layout custom control and the specific left area custom control to the left panel editable area. 

 

 To change the Look and feel create a theme that extends the OneUI theme being used or create a CSS in the application the overrides the classes being implemented.