How to display elements responsively side by side using Bartik theme, similar to bootstrap grid?
I ended up working around it by extracting the relevant css classes of bootstrap into my custom css file. Now I can use the bootstrap grid without disturbing the normal Drupal theme:

.row {
  margin-right: -15px;
  margin-left: -15px;
  box-sizing: border-box;
}
.row:before, .row:after {
  display: table;
  content: " ";
}
.row:after {
  clear: both;
}

.col-md-1, .col-md-2, .col-md-3, .col-md-4, .col-md-5, .col-md-6, .col-md-7, .col-md-8, .col-md-9, .col-md-10, .col-md-11, .col-md-12 {
  position: relative;
  min-height: 1px;
  padding-right: 15px;
  padding-left: 15px;
  float: left;
  box-sizing: border-box;
}

.col-md-12 {
  width: 100%;
}
.col-md-11 {
  width: 91.66666667%;
}
.col-md-10 {
  width: 83.33333333%;
}
.col-md-9 {
  width: 75%;
}
.col-md-8 {
  width: 66.66666667%;
}
.col-md-7 {
  width: 58.33333333%;
}
.col-md-6 {
  width: 50%;
}
.col-md-5 {
  width: 41.66666667%;
}
.col-md-4 {
  width: 33.33333333%;
}
.col-md-3 {
  width: 25%;
}
.col-md-2 {
  width: 16.66666667%;
}
.col-md-1 {
  width: 8.33333333%;
}