Have you added some new Presets, or are you just trying out the one in Boost theme?
Thanks for the repply. I added a preset of the recommended SCSS (Kraus-dark) and at the moment of saving the changes it does not change anything.
I am not sure as I cannot access my computer right now, but looking at the preset it refers to Moodle 2.0 which is not correct as Boost was introduced in Moodle 3.0.
There is a preset called Morecandy, which was the first preset I made for Moodle 3.1 and will not work in Moodle 3.6 because some code has vpchanged and blocks are different, and I have not updated that file.
So the Kruss Boost Dark Preset may not be correct for 3.6 version of Moodle.
Sorry I can’t be if more help.
Looking at the scss for that preset, I'd agree with Mary that it is for an older version of Boost/Moodle
Older presets tend to have the lines
// Import everything.
while newer ones have the lines
// Import FontAwesome.
// Import All of Bootstrap
// Import Core moodle CSS
The newer ones also often have a theme colors array (though this is not essential, but does make some of the writing easier later on as SASS allows you to loop through them)
Sometimes it can be as simple to update as replacing the imports - BUT THAT IS NOT GUARANTEED! It really does depend what is in the preset, but its certainly a place to start. There were many changes during the development of Bootstrap4 and converting presets is entirely dependent on how the preset was written in the first place.