Frederick, I will be giving a presentation at MountainMoot and MoodleMoot US on this topic. I don't have my presentation together yet, but let me try to give you some guidance.
1) Remove what you have done and purge your caches, as suggested by Hartmut.
2) Make a copy of default.scss, found in PathToMoodle/themes/boost/scss/preset. You can rename this copy to anything, such as "fs_preset01.scss".
3) Now go into your moodle, site admin, appearance, boost. Move your fs_preset01.scss file into the General Setting, Additional theme, then Save Changes. While still on this screen, in the Theme Preset dropdown, pick fs_preset01.scss, then Save Changes again. Purge caches. Your boost should not look any different (because your fs_preset01.scss is identical to the default.)
4) Now for the fun. Edit your fs_preset01.scss and make a simple change. For example, change the line "$brand-primary: #00ff00 !default; " This makes the brand "Green". Now, go back into boost, delete your fs_preset01.scss present, reload your edited fs_preset01.scss file, save changes, purge cache, and finally you will see the green.
Yep, wow, a lot of stuff that you have to do, but this is how I have had success using my own preset file.
But here is the real thing that I do. I first experience with CSS changes by going to Boost, Advances Settings tab, and putting desire changes into "Raw SCSS" textbox. This textbox provides me a little easier place to make changes. Then, with time, I move these css changes into my preset file.
In general, I like having my own preset file. But it is a little hard to understand all of the things built into the default, so I actually prefer working in Raw SCSS textbox. But I do understand the value of having your own default preset. The concept is that I could give you mine, mean one file, and your moodle would look like mine.
For whatever reason, this Boost theme takes awhile to refresh (when making changes and purging caches.)
I am going to attach my experimental fs_preset01.scss file with this one change to Green.
I am not an expert at any of this. I have learned from some great folks here on moodle.org.