/// Style element as a circle with optional background and border.
///
/// @param {color} $background - If given it will be set as the background color.
/// @param {border} $border - If given it will be set as the border.
/// @param {length} $size - If given it will set width/height (diagonal, not radius).
@mixin circle($background: false, $border: false, $size: false) {
    border-radius: 50%;

    @if $background {
        background-color: $background;
    }

    @if $border {
        border: $border;
    }

    @if $size {
        width: $size;
        height: $size;
    }
}