MDL-69087 added an option to let themes personalize the H5P styles.
This patch implements the h5p_alter_styles() method in boost, to
let admins personalize the H5P styles using the 'Raw initial SCSS'
and 'Raw SCSS' theme settings.
That way, users won't need to create their own themes to define
some styles for the H5P player.
Existing mod_hvp pluging has a renderer to let Moodle instances
alter styles, Javascript, semantics (fields in the editor) and
content: https://github.com/h5p/moodle-mod_hvp/blob/stable/renderer.php
The approach for core_h5p is exactly the same, to let people
to reuse their existing code.