Change fonts Course Presentation

I’m trying, without success, to change the font used for text inside the “course presentation” H5P activity.What I did :  
  • follow the visual changes guide ( I was successful into changing the fonts when in Edit mode, but the change disappear when quitting Edit.
  • add custom css to the page, addressing the element class (.h5p-course-presentation .h5p-element-inner.h5p-advanced-text) : no joy
  • install “Fondation theme”, which allows to add custom CSS specifically for H5P, and paste the CSS into the appropriate field : no joy
  When inspecting the element (text) I want to modify, I found the property to modify (see image in attachement). But as I guess form the file name, this file is generated on the fly, but I don’t know where to find the code it is based on. This could help me understand the generation process and find a solution.  Has anyone pointers for showing me the right direction please ?