disabled custom checkbox with gradients is not grayed out in checked state

Created by: kkozlik

If the custom checkbox is in checked state and if gradients are enabled it has a same look whether it is enabled or disabled.

Can't reproduce a codepen example, because I can't enabled gradients without compiling the scss.

I think the solution might be adding:

background-image:none;

to scss/_custom-forms.scss line 138

Tested with: