General developer forum

Hi,
I am completely new to Moodle code. I want to change the CSS of the theme Eguru by creating a child theme using the steps of this tutorial https://docs.moodle.org/dev/Creating_a_theme_based_on_boost#Duplicate_the_settings_from_Boost. The tutorial is based on the theme Boost but I thought I could apply the same logic.

Unfortunately, the child theme I created doesn't inherit the features of the parent Eguru. I didn't overwrite any function, the purpose of the child theme is just to add custom CSS.
What is the best way to achieve this?
Eguru is a child theme of Boost. Is it possible to create a child theme from another child theme?
Is there any easier way to add custom CSS without creating a child theme?

My Moodle version is 4.2.1
Eguru version v4.2.1    2023080101