Main Container Columns' Width

Hi
I am just a newbie with Drupal and I am currently experimenting with 'Drupal8 W3CSS Theme' and I have one problem I cannot solve [well two really!].
On the Drupal8 W3CSS Theme Configuration page [ https://www.drupal.org/docs/8/themes/drupal8-w3css-theme-configuration → ] there is a section on ' How to change website width ' that shows a input field where you can manually set the 3 columns width. Well I have installed it and all works fine on Drupal 8, but this field is not present and I am forced to use the 2 preset sizes of 25-50-25 or 33-33-33 !

I have 2 questions really:

1. how can I change the settings page to have the input field and where?
2. which CSS file I can change manually to do that [as well] ? Could not find it.

Thank you in advance to whoever can help me out

M