From the Sass language reference documentation:
Boolean Operations
SassScript supports
and,or, andnotoperators for boolean values.
(link)
So an if statement expression with a compound condition would look like this:
@if $var1 == value1 and $var2 == value2 {
// ...
}
Further, parentheses can be used to affect the order of operations in a more complicated expression:
@if ($var1 == value1 and not ($var2 == value2)) or ($var3 == value3) {
// ...
}