Frame managers are responsible for controlling the look and feel of an application frame. Each different kind of appearance, whether it be Motif or Microsoft Windows, is expressed by a different frame manager. CLIM provides frame managers that interface to a large number of host environments, including X Windows. There is also a "generic" frame manager that allows applications to maintain a "CLIM look and feel" across all platforms, rather than adopting the style of the underlying windowing system. Existing frame managers can be customized, or entirely new frame managers can be created to give your application the look and feel you desire.
A frame manager is responsible for interpreting the portable, window-system-independent layout specification of an application frame in the context of the look and feel supported by the frame manager. The abstract gadget panes, such as the scroll bars and buttons, will be mapped into specific pane classes that implement the gadget in terms of the native gadget of the host window system. For example, scroll-bar is mapped onto internal classes which use the SCROLLBAR control on Microsoft Windows and the ScrollBar widget on Motif.
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017