Just to let you know. When adding the code to the addon's Custom Css field, at the output an addon ID is automatically added to this code.
For clarity, I will add class to addon.

In the source code you can see the entire addon and the place where my-Сlass was added.

In the addon's Custom Css field you can use code for my-Сlass and all its children.
The code for the parental elements of my-Сlass like #sppb-addon-xxxxxxxx & #sppb-addon-wrapper-xxxxxxxx should be added to a more global level but not to addon's Custom Css field.