Need to find a way to synchronize the preferences with the default values of the render window widgets of the multi widget.
Problem: Preferences are used in the preference manager, which has no access to the multi widget (editor).

Having a dynamically adjustable number of render windows inside the multi widget editor, it is not reasonable to allow background- and decoration-color changes inside the preferences for each render window individually.
Instead a color-list / color-map could be used that predefines some color-settings for an arbitrary number of render windows.
Changing the colors / decorations individually for a render window happens somewhere else (e.g. via a button in the render window or the multi widget itself).

Idea:

• Use color-lists, that are selectable in the preference page.
• Add checkbox to enable / disable render window specific decoration buttons. These buttons allow to set the decorations for each render window individually.

First draft of a decoration manager was added; currently only a black-and-white color style is available. Render window individual decorations are currently not implemented.

Why don't you let developers of new editors take care if they have or need preference settings? Just give them a usable interface to modify windows to their likeing. Where the information comes from is not our business.

Or was your propsal just for the our new editor? Thank I think it is a pragmatic approach.

This was a proposal for the new editor, since it might contain too many render windows to change the decoration color for each render window via the preference page.
However, this decoration manager can be used to set the colors and decorations 'OnPreferencesChanged'.

I just tested the latest mxn multi widget on develop and now the render window menu icons have a thick border around them - which does not look nice. Need to find out how this happened.

This seemed to be introduced / not completely fixed with fbc49fc5c831. Fixed it by adding the QmitkRenderWindowWidget class name to explicitly define the border only for this class (as it was already partially done in the mentioned commit).