// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
//
// Copyright (c) 2015-2016 Andrey Antukh <niwi@niwi.nz>
// Copyright (c) 2015-2016 Juan de la Cruz <delacruzgarciajuan@gmail.com>

// Padding & Margin sizes
$x-small: 5px;
$small: 10px;
$medium: 15px;
$big: 20px;
$x-big: 30px;

// Border radius
$br-small: 3px;
$br-medium: 5px;
$br-big: 8px;

// Alignments
.text-left {
    text-align: left;
}

.text-right {
    text-align: right;
}

.row-flex {
  align-items: center;
  display: flex;
  margin-bottom: $x-small;

  &.column {
    flex-direction: column;
  }

}

&.half {
  width: 50%;
}

// Display
.hidden {
    display: none;
}

.hide {
    opacity: 0;
}

.display {
    opacity: 1 !important;
}

// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
//
// Copyright (c) 2015-2016 Andrey Antukh <niwi@niwi.nz>
// Copyright (c) 2015-2016 Juan de la Cruz <delacruzgarciajuan@gmail.com>

// Padding & Margin sizes
$x-small: 5px;
$small: 10px;
$medium: 15px;
$big: 20px;
$x-big: 30px;

// Border radius
$br-small: 3px;
$br-medium: 5px;
$br-big: 8px;

// Alignments
.text-left {
    text-align: left;
}

.text-right {
    text-align: right;
}

.row-flex {
  align-items: center;
  display: flex;
  margin-bottom: $x-small;

  &.column {
    flex-direction: column;
  }

  &.center {
    justify-content: center;
  }
}

&.half {
  width: 50%;
}

// Display
.hidden {
    display: none;
}

.hide {
    opacity: 0;
}

.display {
    opacity: 1 !important;
}

Add .center helper class as friend of .row-flex
Add .center helper class as friend of .row-flex
