In theme/boost/scss/moodle/variables.scss, some variables can't be overwritten

Steps to reproduce: 
1. Login as Admin 
2. Go to Site administration -> your Theme 
3. Add "$activity-iconcontainer-height: 70px; $activity-iconcontainer-width: 70px;" to the Raw initial SCSS.
4. Click "Save changes"
5. Go to a course
6. Turn editing on
7. Click on "Add an activity or resource"

Actual result:
The size of an activity container remains unchanged

Expected result:
Activity container has width and height of 70px