Custom checkboxes and radios retheming

Created by: MartijnCuppens

The custom checkboxes and radios are cool, but I think they might need some retheming. I don't think it's a good idea to make them gray, it looks like they are disabled and there isn't much of a difference between the disabled and not disabled versions.

Imo they should have the same background and border color as the .form-control elements and also take over the theming of disabled state.

I made a demo of the rethemed custom checkboxes here, it's a part from the forms documentation page which also includes the validated theming:

V1 (same border color as .form-control, $gray-400): https://codepen.io/MartijnCuppens/pen/KBYKKz V2 (darker borders, $gray-500): https://codepen.io/MartijnCuppens/pen/MPYrmo