Set $input-plaintext-color variable

Author: Carl-Hugo

scss/_dark-variables.scss

@@ -134,6 +134,8 @@ $input-focus-bg: $body-bg;
 $input-focus-border-color: lighten($component-active-bg, 25%);
 $input-focus-color: $input-color;
 
+$input-plaintext-color: $body-color;
+
 $input-group-addon-color: $input-color;
 $input-group-addon-bg: $gray-800;
 $input-group-addon-border-color: $input-border-color;