polishing of sidebar in public view

[szaimen]

just tested on nc29.0.0b1

• The icons in system dark mode in the header look broken:
  
• Also the sidebar toggle button does not seem to work... fix(PublicShareSidebar): fix prop reactivity #12133

[Pytal]

Seems to be the Talk public share sidebar, cc @danxuliu

[nickvergessen]

Reopening as the icon color and file name color is still wrong if I see that correctly

[nickvergessen]

And just for the record, we need to adjust the used variable, not the content of the variable. it claims to be a button tertiary-on-primary, but it's not on primary, it's on the background.
Might be non trivial to solve. Best is to check the color/variable used by the app list when you are logged in