[variables] input-bg and input-color conflict

Created by: arthurshlain

You must use both $body-bg and $body-color values or $white and $black as default, because now we have unreadable text after changing $body-bg and $body-color values between themselves.

Current values:

$input-bg:                              $white !default; // hardcoded by default
$input-disabled-bg:                     $gray-200 !default;
$input-disabled-border-color:           null !default;

$input-color:                           $body-color !default; // cause unreadable text if $body-color = #FFFFFF
$input-border-color:                    $gray-400 !default;
$input-border-width:                    $input-btn-border-width !default;
$input-box-shadow:                      $box-shadow-inset !default;