When you go to ‘Themes and components’, and then ‘Edit Code’ in the Custom code section, it’ll take you to edit code.
The back actions will take you to the old UI (admin/customize/themes/28), rather than the new one (admin/config/customize/components)
Hi, thanks for the report.
Ok I see what you mean I think the URL is correct, but yes it’s showing the sidebar which it shouldn’t:
What I should see:
What you get after pressing back from edit code:
I can repro the issue in the OP too. 
I don’t think the old style of the theme/theme component page should be accessible through a UI link anywhere? I think they should all point to the new updated ‘config’ version.
I think the links on the admin warnings also send you to the old style as well.
Couple of screenshots for comparison:
New:
Old:
ETA:
Ah, I may have misunderstood what you meant when you said the links were right. I’m not sure. 
If so, the only useful info in this post may be that this also happens for the admin warnings on the dashboard too.
Cross-linking another post that reports it Introducing a new visual layout for themes and theme components - #19 by Moin
For me there is no need to edit custom css/js. Just opening it is enough, if there is any code.
(I actually used it yesterday. We don’t have any easy way to import/export theme components is bulk to new forum. Old menu is very much easier to copy and then create in the new forum 
)
Hi @noahl just letting you know sorting this out is high on our list, expect an update on it either late this week or early next.
