Move theme toolbar CSS to the Seven theme

Neds issue summary as

• This one refers to Seven, a now-removed theme
• There's mention of "Add a library alter hook to load the admin theme CSS". The claro theme now styles the toolbar on the admin & default themes. We should check if that change meets that specific need #3070493: Introduce a mechanism to provide an alternate Claro design for the toolbar in the future →
• Part of the justification to doing this is adhering to as-documented CSS standards Our CSS standards define module CSS as: “the minimal styles needed to get the module's functionality working.”
  Theme CSS is defined as: “extra styles to make the module's functionality aesthetically pleasing. ”. Given this issue is 8+ years old, we should establish that this is still worth pursuing even if it might disrupt the way many sites are accustomed to.

It has been two years since more information was asked for about this issue and the last work here was 9 years ago. Since justification for keeping this open has not been given I am closing this.

Thanks to everyone who helped here.