Here is the .scss I copy and paste to every single new project I ever create!

_functions.scss

@function rem($pixels, $base: 16) {
    @return #{$pixels/$base}rem;
}

@function em($pixels, $base: 16) {
    @return #{$pixels/$base}em;
}

_variables.scss

// Colors
$white: #fff;
$black: #000;

// fonts
$sans: sans-serif;

// Breakpoints
$smallbp: rem(500); 
$mediumbp: rem(700); 
$largebp: rem(960);
$xlargebp: rem(1100);

_reset-and-defaults.scss

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-size: 100%;
    font-weight: normal;
    line-height: 1.6;
    font-family: $sans;
    text-rendering: optimizeLegibility;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

h1,
h2,
h3,
h4,
h5,
h6 {
    font-size: rem(16);
    line-height: 1.2;
    font-weight: bold;
    font-family: $sans;
}

a {
    text-decoration: none;
}

a,
button,
label,
input[type="submit"],
input[type="checkbox"],
input[type="radio"] {
    cursor: pointer;
}

li {
    list-style: none;
}

input,
select,
button,
textarea {
    appearance: none;
    border: none;
    background: transparent;
    outline: none;
    font-family: $sans;
    font-size: rem(16);
}

img,
svg {
    display: block;
    max-width: 100%;
}

_global-classes.scss

.width {
    width: 90%;
    max-width: rem(1140);
    margin: 0 auto;
}

.clear:after {
    clear: both;
    content: '';
    display: table;
}

.hide {
    display: none;
}

.offscreen {
    left: -999em;
    position: absolute;
}