Ability to set dark mode per dashboard rather than only globally within a space.

[AlonaNadler]

Requested by a user:

We had some dashboards and saved searched that were dark and some that were not, anyway from the point of view of a user, we have some users that are struggling with reading white fonts over a black background, at the same time there are some dashboards that we prefer to have as dark.
Ideally, there would be default and a user could customize their own options, but the next best thing would be to allow per dashboard customization.

[inqueue]

Another user has requested the same. Also in the request was a way to disable the dark them global setting so it cannot be changed by end users.