Don't want to learn Bootstrap classes? Go to Bootstrap builder for busy developers.

Bootstrap class: .border

<span class="border border-dark">Hello World</span>

Preview

Hello World

Sass source

/* _borders.scss:7 */
.border         { border: $border-width solid $border-color !important; }
.border-top     { border-top: $border-width solid $border-color !important; }
.border-right   { border-right: $border-width solid $border-color !important; }
.border-bottom  { border-bottom: $border-width solid $border-color !important; }
.border-left    { border-left: $border-width solid $border-color !important; }

.border-0        { border: 0 !important; }
.border-top-0    { border-top: 0 !important; }
.border-right-0  { border-right: 0 !important; }
.border-bottom-0 { border-bottom: 0 !important; }
.border-left-0   { border-left: 0 !important; }

@each $color, $value in $theme-colors {
  .border-#{$color} {
    border-color: $value !important;
  }
}

Uses variables

/* _borders.scss:7 */
$border-width
$border-color
$border-width
$border-color
$border-width
$border-color
$border-width
$border-color
$border-width
$border-color
$color
$value
$theme-colors
$color}
$value