Custom Predefined Theme

Problem/Motivation

In 8.x-1.* it was possible to enter a custom theme name in the theme settings at /admin/appearance/settings/drupal8_w3css_theme. Since 2.* this is no longer possible, as there is a select field which neither allows the option "custom" nor let the admin specify a custom theme name. In 1.* this was a simple text field, where everything could be entered. I created a custom theme with a special name (for easy recognition), which I cannot select anymore.

Steps to reproduce

Create a custom theme like described in README.txt of the base theme:

* How to create a new custom predefined color theme
  ...................................................

1) Go to https://www.w3schools.com/w3css/w3css_color_themes.asp and create your
custom theme.  Follow the steps to create a private theme.

2) After you finish go to your-drupal-site-directory/themes/drupal8_w3css_theme/
css/w3-css-theme-custom/w3-theme-custom.css Open w3-theme-custom.css and delete
the old code then paste the new code for your private theme.

3) Go to www.your-drupal-site-name/admin/appearance/settings/drupal8_w3css_theme
and click on W3css Predefined Theme and enter w3-theme-custom. How to install
drupal8 w3css subtheme >>  Install W3CSS Theme First, then install
drupal8 w3css subtheme.

It is not possible to enter "w3-theme-custom", as it is a select field.

Proposed resolution

Change back to a text field.