Using multiple conditions (AND) in SASS IF statement

From the Sass language reference documentation:

Boolean Operations

SassScript supports and, or, and not operators 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) {
    // ...
} 

You can also do:

@if index("foo" "bar", $value) { .. }

Beware though:

1. It might not be quite obvious what you are trying to do.
2. It returns a number or null, not a boolean.