You can now find a switch for Dark/Light mode for your own user in the website footer (bottom left of the page).
I'll look into making the option set in the StaffCP as a default for all users in the next release.
Can you please re-upload the custom/panel_templates/Default folder to ensure that it is up to date
You can get it from either the pre-release 12 upgrade files if you still have them, or you can find it in the full pre-release 12 (just make sure you only reupload the panel template and not anything else)
This is already possible using the Latest Posts widget, there's an example on the forum index on this site
Is this what you're looking for?
Please switch to the Default panel template until your custom panel template has updated to pre-release 12.