Allow customization of Admin Color Schemes for dashboard

[Ate Up With Motor]

This issue -- #48585 -- highlights the need for a sorely missed core accessibility feature: the ability for users to customize the preset Admin Color Schemes for the administrative dashboard.

I know there are plugins that can customize the dashboard, but I believe this feature belongs in core for these reasons:

1. It affects users' ability to access and manage the basic functions of any WordPress installation.
2. The necessary styles already exist in core and there is already provision to save specific color scheme settings in the user profile. This functionality would be a refinement of pieces already baked in.
3. The accessibility shortcomings of the default set of Admin Color Schemes are already a known issue.
4. For intermediate users, being able to make certain customization choices to an existing scheme or palette is typically the easiest way to achieve a desired result. (More complex functionality, like altering the button layouts, would probably be best left to plugins aimed at more advanced users.)

As a user, I want to emphasize that color selection for accessibility is very much an individual choice. There are, for example, W3C standards-validated high-contrast color schemes that are very hard on my eyes. Regardless of whatever other changes may be made to the default Admin Color Schemes for accessibility, it would be beneficial to allow users to further tailor those schemes to suit their own needs. Since the Admin Color Scheme choices are saved in the individual user profile and the color choices are not publicly facing, there's no obvious reason why WordPress users shouldn't have that option.

Ideally, such a customization option would include some kind of color picker interface. In a pinch, providing a hyperlink to an external color chooser with clear instructions to copy and paste the hexadecimal code for the desired color into the appropriate spot would probably be adequate and would still be a significant improvement.

I'm not a developer, so the engineering of such a feature is beyond my scope, but I figured it was worth asking.

[afercia]

This ticket was discussed during today's accessibility bug-scrub. The idea sounds sensible and worth considering for the future. A lot of preliminary work would need to be done to build such a new feature. In the meantime, a reconsidering the purpose of the preset admin color schemes sounds a more feasible approach.

[melchoyce]

@helen has a plugin which does this, but it's out of date and would need some fixing if we wanted to use it as a base: ​https://wordpress.org/plugins/admin-color-schemer/