Improve the styling of color input elements in the admin theme

Description of the need When a contributed module attempts to add a color input, the current styling in Seven causes it to look very weird. Because of this, any module that has one will need to provide it's own styling of the form.

Proposed solution We can move some of the CSS styles from the Color module to Seven, and that should provide a reasonable base for module developers to use.

PR by @hosef - https://github.com/backdrop/backdrop/pull/2947