CSS

scss !default

You can assign to variables if they aren’t already assigned 
by adding the !default flag to the end of the value. 
This means that if the variable has already been assigned to, 
it won’t be re-assigned, but if it doesn’t have a value yet, it 
will be given one.

what is !default in sass

!default is a Sass flag that indicates conditional assignment 
to a variable — it assigns a value only if the variable 
was previously undefined or null. Example:

$variable: 'test' !default;

To the Sass compiler, this line says:

Assign $variable to value 'test', but only if $variable 
is not already assigned.