Problem/Motivation
[#2847522} allows the user to choose the renderer for the settings tray but in order to sow up in the select the theme must have a library for the offcanvas tray.
Proposed resolution
Add an offcanvas library to Seven theme with styles that follow the Seven style guide.
Remaining tasks
Create patch
User interface changes
When Seven is selected for 'offcanvas admin renderer" the tray will be styled like seven theme
API changes
None
Data model changes
None