custom theme

Hi,

I have just installed the security update and updated from 7.x-4.0 to 7.x-4.9.

On the 7.x-4.0 version, I had set up a custom theme, and it pulled in my custom css.

In 7.x-4.9 the module setting has:
Choose the theme to use for autocompletion dropdown popup. Read documentation to learn how to make your own.
which links to: https://drupal-addict.com/nos-projets/search-autocomplete

And my original custom theme is not listed

But, I don't seem to find the relevant instructions on how to add a custom theme?
The closest I could find was:
https://drupal-addict.com/nos-projets/search-autocomplete/generale-confi...

Custom your own CSS

but it says:
This documentation need further work, please come back later.

Please cna you let me know how to add my custom theme back in?

thank you for your help,
Ic70