Responsive defaults variables

variables used to define responsive

`$gbc-responsive-small-width`

Maximum width for phones / small screen

Type: string

Default value: "576px"

Computed value: "576px"

Alias (for compatibility): $responsive-small-width

Maximum width for tablets / medium screen

Type: string

Default value: "768px"

Computed value: "768px"

Alias (for compatibility): $responsive-medium-width