How to override custom checkout less variables?

In Magento2 I’ve created a new theme for the checkout, with Magento/blank as parent.
I want to start editing the style for that page, so I’ve created a _theme.less file in app/design/frontend/<Vendor>/<module_checkout>/Magento_Checkout/web/css/source/_theme.less.

The idea was to override all the variables I needed to. I try to edit some variables, but it’s hard to find them.

But if I try to set one of them in _theme.less with a different value, it does not update on the page.
Even after
rm -r var/view_preprocessed/
or
rm -r pub/static/

For example if I wanted to change the checkout steps to be 33.3% of the width instead of this:

Any ways I can afford that? I just need to understand where the variables to change are, and how to change them.

Thanks guys!