Enable custom CSS in the CP.

[murtaugh]

A native mechanism for adding custom CSS to the CP would go a long way to allowing people to test and distribute solutions to #1590.

Once the general accessibility issues are addressed, the same custom CSS feature would allow for the distribution of CSS that addresses individual accessibility issues (example: CSS files tuned for specific types of color blindness).

[robsonsobral]

There's an add-on for that, but I think it's too powerful. It should be something configurable only by editing config.php.